nullfs: ensure correct lock is taken after bypass.

If lower VOP relocked the lower vnode, it is possible that nullfs
vnode was reclaimed meantime. In this case nullfs vnode no longer
shares lock with lower vnode, which breaks locking protocol.

Check for the condition and acquire nullfs vnode lock if detected.

Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week