disposing fifo resources in fifo_cleanup() instead using of
"vp->v_usecount == 1". There may be other references to the vnode, for
instance by nullfs, at the time fifo_open() or fifo_close() is called,
which could cause a resource leak.
Don't bother grabbing the vnode interlock in fifo_cleanup() since it no
longer accesses v_usecount.
95c6cd2f4b