MFC 272550:

  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.