- Add the tunable and sysctl for the mpsafevfs. It currently defaults

 - Add the tunable and sysctl for the mpsafevfs.  It currently defaults
   to off.
 - Protect access to mnt_kern_flag with the mointpoint mutex.
 - Remove some KASSERTs which are not legal checks without the appropriate
   locks held.
 - Use VCANRECYCLE() rather than rolling several slightly different
   checks together.
 - Return from vtryrecycle() with a recycled vnode rather than a locked
   vnode.  This simplifies some locking.
 - Remove several GIANT_REQUIRED lines.
 - Add a few KASSERTs to help with INACT debugging.

Sponsored By:	Isilon Systems, Inc.