MFC r304184:

MFC r304184:

sem_post(): wake up the sleeper only after adjusting has_waiters

If the caller of sem_post() wakes up a thread sleeping via sem_wait()
before it clears the has_waiters flag, the caller of sem_wait() has no way of
knowing when it is safe to destroy the semaphore and reuse the memory. This is
because the caller of sem_post() may be interrupted between the wake step and
the clearing of has_waiters. It will then write into the has_waiters flag in
userspace after being preempted for some unknown amount of time.

Approved by:	vangyzen (mentor)
Sponsored by:	Dell Inc.

git-svn-id: svn://svn.freebsd.org/base/stable/10@305063 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f