MFC r311172,r311194,r311226,r312389,r312390:

mtx: reduce lock accesses

Instead of spuriously re-reading the lock value, read it once.

This change also has a side effect of fixing a performance bug:
on failed _mtx_obtain_lock, it was possible that re-read would find
the lock is unowned, but in this case the primitive would make a trip
through turnstile code.

This is diff reduction to a variant which uses atomic_fcmpset.

Reduce lock accesses in thread lock similarly to r311172

mtx: plug open-coded mtx_lock access missed in r311172

rwlock: reduce lock accesses similarly to r311172

sx: reduce lock accesses similarly to r311172