kernel - Fix dangling ref on fifo vnode

kernel - Fix dangling ref on fifo vnode

* Fix dangling refs on the fifo vnode which prevent unmount from
  succeeding.

* Fix open/close races which can occur during fifo open and close
  due to blocking conditions.  Use a lockmgr lock instead.

* Retain use of vp->v_token to help interlock read, write, and
  ioctl operations.

* This situation occurs when a filesystem fifo (aka via mkfifo)
  blocks on open() and is then interrupted.