FBSD: band-aid vnode locking in lookup

FBSD: band-aid vnode locking in lookup

The lock order requires that we acquire vnode locks from the root
towards the leaf.  When looking up "..", this requires that we
unlock the directory before locking the child, otherwise we
are susceptible to deadlock.
This is only a band-aid, as afs_vop_lookup should be rewritten.

Change-Id: Ie99a677978370488f8edc7128014b89abc00a56c
Reviewed-on: http://gerrit.openafs.org/3035
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>