FBSD: adhere to gop_lookupname() semantics

The current semantics are that gop_lookupname() returns an unlocked
vnode; the previous code was written to a different semantic that
a locked vnode should be returned.

This makes a disk cache more likely to work on FreeBSD, but such
configurations remain not very tested.

Change-Id: I12ac77cd271be72af2fa4045c2ebf576847b625e
Reviewed-on: http://gerrit.openafs.org/11317
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>