dmsg - Stabilization work

dmsg - Stabilization work

* Add a refs field to dmsg_state and kdmsg_state for retention and
  disposal.

* Separate out the tracking of state->subq and the state RB trees.

* Greatly simplify the iocom shutdown code and functions related
  to handling communications failures.  When iterating states for
  shutdown, which requires simulating a received failure message,
  we can now simply iterate via state->subq.

* Greatly simplify how the simulated failures are generated and
  handled (See dmsg_msg_simulate_failure()).  This is probably the
  most complex part of the library.

* Fix a memory leak in kern_dmsg.c.

* Replace xdisk's per-softc token with a lockmgr lock.  Atomicy
  has to be guaranteed across blocking conditions in certain cases
  and it was easier to simply use a lock for everything.

  Cleanup the locking.

* Ripout the shutdown check in the I/O path, which can deadlock
  the disk management threads.  This will need to be revisited
  as it means that /dev/xa* and /dev/serno* devices remain in
  /dev after a link failure even when there are no opens on the
  device.

* Add the B_FAILONDIS flag to struct buf.  This flag allows the disk
  probe code to tell xdisk that it is ok for the I/O to fail, allowing
  xdisk to discard I/Os that would otherwise block or deadlock the
  disk probe code when the related network connection is lost.