thread: Simplify sanitizer integration with thread creation

thread: Simplify sanitizer integration with thread creation

fork() may allocate a new thread in one of two ways: from UMA, or cached
in a freed proc that was just allocated from UMA.  In either case, KASAN
and KMSAN need to initialize some state; in particular they need to
initialize the shadow mapping of the new thread's stack.

This is done differently between KASAN and KMSAN, which is confusing.
This patch improves things a bit:
- Add a new thread_recycle() function, which moves all kernel stack
  handling out of kern_fork.c, since it doesn't really belong there.
- Then, thread_alloc_stack() has only one local caller, so just inline
  it.
- Avoid redundant shadow stack initialization: thread_alloc()
  initializes the KMSAN shadow stack (via kmsan_thread_alloc()) even
  through vm_thread_new() already did that.
- Add kasan_thread_alloc(), for consistency with kmsan_thread_alloc().

No functional change intended.

Reviewed by:	khng
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D44891