MFC 208915,208991:

MFC 208915,208991:
- Use a bit more care when moving I/O APIC interrupts between CPUs.  Mask
  the interrupt followed by a brief delay if it is not currently masked
  before moving the interrupt.
- Move the icu_lock out of ioapic_program_intpin() and into callers.  This
  closes a race where ioapic_program_intpin() could use a stale value of
  the masked state to compute the masked bit in the register.

git-svn-id: svn://svn.freebsd.org/base/stable/8@209506 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f