null_lookup: restore dvp lock always, not only on success

Caller of VOP_LOOKUP() passes dvp locked and expect it locked on return.
Relock of lower vnode in any case could leave upper vnode reclaimed and
unlocked.

Reported and tested by: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31310