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.
MFC r273129:
Implement FIODTYPE for master ptys.
MFC r273130:
Change the deadfs poll VOP to return POLLIN|POLLRDNORM if the caller
is interested in i/o state.  Return POLLNVAL for invalid bits, similar
to poll_no_poll().
MFC r273131:
When vnode bypass cannot be performed on the cdev file descriptor for
read/write/poll/ioctl, call standard vnode filedescriptor fop.
MFC r273132:
Style changes for deadfs.