Remove stray uma_mtx lock/unlock in zone_drain_wait()

Callers of zone_drain_wait(M_WAITOK) do not need to hold (and were not)
the uma_mtx, but we would attempt to unlock and relock the mutex if we
had to sleep because the zone was already draining. The M_NOWAIT callers
may hold the uma_mtx, but we do not sleep in that case.

Reviewed by:	jhb
MFC after:	3 days


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272550 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f