rdmsr_safe/wrmsr_safe: handle pcb_onfault nesting

rdmsr_safe/wrmsr_safe: handle pcb_onfault nesting

rdmsr_safe and wrmsr_safe can be called while pcb_onfault is already
set, so the functions are modified to preserve the handler rather than
resetting it before returning.

One case where that happens is when AMD microcode update routine
is executed on a stack where copyin / copyout was already active.

Here is a sample panic message from a crash caused by resetting the
handler:

  <118>Updating CPU Microcode...

  Fatal trap 12: page fault while in kernel mode
  cpuid = 3; apic id = 03
  fault virtual address   = 0x11ed0de6000
  fault code              = supervisor write data, page not present
  instruction pointer     = 0x20:0xffffffff80c2df03
  stack pointer           = 0x28:0xfffffe01ce4a4c70
  frame pointer           = 0x28:0xfffffe01ce4a4c70
  code segment            = base 0x0, limit 0xfffff, type 0x1b
                          = DPL 0, pres 1, long 1, def32 0, gran 1
  processor eflags        = interrupt enabled, resume, IOPL = 0
  current process         = 117 (logger)
  trap number             = 12
  panic: page fault
  cpuid = 3
  time = 1681462027
  KDB: stack backtrace:
  db_trace_self_wrapper() at 0xffffffff80615deb = db_trace_self_wrapper+0x2b/frame 0xfffffe01ce4a4830
  kdb_backtrace() at 0xffffffff80943c77 = kdb_backtrace+0x37/frame 0xfffffe01ce4a48e0
  vpanic() at 0xffffffff808f5fe5 = vpanic+0x185/frame 0xfffffe01ce4a4940
  panic() at 0xffffffff808f5da3 = panic+0x43/frame 0xfffffe01ce4a49a0
  trap_fatal() at 0xffffffff80c31849 = trap_fatal+0x379/frame 0xfffffe01ce4a4a00
  trap_pfault() at 0xffffffff80c318b5 = trap_pfault+0x65/frame 0xfffffe01ce4a4a60
  trap() at 0xffffffff80c30f5f = trap+0x29f/frame 0xfffffe01ce4a4b80
  trap_check() at 0xffffffff80c31c29 = trap_check+0x29/frame 0xfffffe01ce4a4ba0
  calltrap() at 0xffffffff80c07fd8 = calltrap+0x8/frame 0xfffffe01ce4a4ba0
  --- trap 0xc, rip = 0xffffffff80c2df03, rsp = 0xfffffe01ce4a4c70, rbp = 0xfffffe01ce4a4c70 ---
  copyout_nosmap_std() at 0xffffffff80c2df03 = copyout_nosmap_std+0x63/frame 0xfffffe01ce4a4c70
  uiomove_faultflag() at 0xffffffff8095f0d5 = uiomove_faultflag+0xe5/frame 0xfffffe01ce4a4cb0
  uiomove() at 0xffffffff8095efeb = uiomove+0xb/frame 0xfffffe01ce4a4cc0
  pipe_read() at 0xffffffff80968860 = pipe_read+0x230/frame 0xfffffe01ce4a4d30
  dofileread() at 0xffffffff809653cb = dofileread+0x8b/frame 0xfffffe01ce4a4d80
  sys_read() at 0xffffffff80964fa0 = sys_read+0xc0/frame 0xfffffe01ce4a4df0
  amd64_syscall() at 0xffffffff80c3221a = amd64_syscall+0x18a/frame 0xfffffe01ce4a4f30
  fast_syscall_common() at 0xffffffff80c088eb = fast_syscall_common+0xf8/frame 0xfffffe01ce4a4f30
  --- syscall (3, FreeBSD ELF64, read), rip = 0x11ece41cfaa, rsp = 0x11ecbec4908, rbp = 0x11ecbec4920 ---
  Uptime: 41s

And another one:

  Fatal trap 12: page fault while in kernel mode
  cpuid = 4; apic id = 04
  fault virtual address   = 0x800a22000
  fault code              = supervisor write data, page not present
  instruction pointer     = 0x20:0xffffffff80b2c7ca
  stack pointer           = 0x28:0xfffffe01c55b5480
  frame pointer           = 0x28:0xfffffe01c55b5480
  code segment            = base 0x0, limit 0xfffff, type 0x1b
                          = DPL 0, pres 1, long 1, def32 0, gran 1
  processor eflags        = interrupt enabled, resume, IOPL = 0
  current process         = 68418 (pfctl)
  trap number             = 12
  panic: page fault
  cpuid = 4
  time = 1625184463
  KDB: stack backtrace:
  db_trace_self_wrapper() at 0xffffffff805c1e8b = db_trace_self_wrapper+0x2b/frame 0xfffffe01c55b5040
  kdb_backtrace() at 0xffffffff808874b7 = kdb_backtrace+0x37/frame 0xfffffe01c55b50f0
  vpanic() at 0xffffffff808449d8 = vpanic+0x188/frame 0xfffffe01c55b5150
  panic() at 0xffffffff808445f3 = panic+0x43/frame 0xfffffe01c55b51b0
  trap_fatal() at 0xffffffff80b300a5 = trap_fatal+0x375/frame 0xfffffe01c55b5210
  trap_pfault() at 0xffffffff80b30180 = trap_pfault+0x80/frame 0xfffffe01c55b5280
  trap() at 0xffffffff80b2f729 = trap+0x289/frame 0xfffffe01c55b5390
  trap_check() at 0xffffffff80b304d9 = trap_check+0x29/frame 0xfffffe01c55b53b0
  calltrap() at 0xffffffff80b0bb28 = calltrap+0x8/frame 0xfffffe01c55b53b0
  --- trap 0xc, rip = 0xffffffff80b2c7ca, rsp = 0xfffffe01c55b5480, rbp = 0xfffffe01c55b5480 ---
  copyout_nosmap_std() at 0xffffffff80b2c7ca = copyout_nosmap_std+0x15a/frame 0xfffffe01c55b5480
  pfioctl() at 0xffffffff85539358 = pfioctl+0x4d28/frame 0xfffffe01c55b5940
  devfs_ioctl() at 0xffffffff807176cf = devfs_ioctl+0xcf/frame 0xfffffe01c55b59a0
  VOP_IOCTL_APV() at 0xffffffff80bb26e2 = VOP_IOCTL_APV+0x92/frame 0xfffffe01c55b59c0
  VOP_IOCTL() at 0xffffffff80928014 = VOP_IOCTL+0x34/frame 0xfffffe01c55b5a10
  vn_ioctl() at 0xffffffff80923330 = vn_ioctl+0xc0/frame 0xfffffe01c55b5b00
  devfs_ioctl_f() at 0xffffffff80717bbe = devfs_ioctl_f+0x1e/frame 0xfffffe01c55b5b20
  fo_ioctl() at 0xffffffff808abc6b = fo_ioctl+0xb/frame 0xfffffe01c55b5b30
  kern_ioctl() at 0xffffffff808abc01 = kern_ioctl+0x1d1/frame 0xfffffe01c55b5b80
  sys_ioctl() at 0xffffffff808ab982 = sys_ioctl+0x132/frame 0xfffffe01c55b5c50
  syscallenter() at 0xffffffff80b30cc9 = syscallenter+0x159/frame 0xfffffe01c55b5ca0
  amd64_syscall() at 0xffffffff80b309a5 = amd64_syscall+0x15/frame 0xfffffe01c55b5d30
  fast_syscall_common() at 0xffffffff80b0c44e = fast_syscall_common+0xf8/frame 0xfffffe01c55b5d30

PR:		276426
Reviewed by:	kib, markj
Approved by:	re (cperciva)

(cherry picked from commit 486b265a8fb6b2aad37f2819fa04feacf8184d53)