MFC r272573:

  Change the UMA mutex into a rwlock

  Acquire the lock in read mode when just needed to ensure the stability
  of the keg list. The UMA lock may be held for a long time (relatively
  speaking) in uma_reclaim() on machines with lots of zones/kegs. If the
  uma_timeout() would fire during that period, subsequent callouts on that
  CPU may be significantly delayed.