unionfs: various locking fixes

--Clearing cached subdirectories in unionfs_noderem() should be done

under the vnode interlock

--When preparing to switch the vnode lock in both unionfs_node_update()

and unionfs_noderem(), the incoming lock should be acquired before
updating the v_vnlock field to point to it.  Otherwise we effectively
break the locking contract for a brief window.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D32629