kernel - Rename desiredvnodes to maxvnodes, fix deadlock

kernel - Rename desiredvnodes to maxvnodes, fix deadlock

* Rename the kernel variable 'desiredvnodes' to 'maxvnodes' to match
  the sysctl name (which has always been 'maxvnodes'), and to make the
  code more readable.

* Probable fix to a rare mount/umount deadlock which can occur in two
  situations (1) When a large number of mounts and unmounts are running
  concurrently, and (2) During a umount -a, shutdown, or reboot.

* Considered minor, normal use cases will not reproduce this bug.  Only
  synth or poudriere can generate the mount/umount traffic necessary to
  reproduce this bug.

* Also fixes a minor kernel memory leak of the mount structure which can
  occur when a 'df' or filesystem sync races a umount.  Also minor.

Reported-by: marino (mount race)