FBSD: remove vestiges of Giant

FBSD: remove vestiges of Giant

We do not support versions of FreeBSD that used Giant to
serialize access to vnode fields, and we always use the same locking
(which really ought to be MPSAFE).  Always set the MPSAFE flags
as needed, and do not bother to call VFS_[UN]LOCK_GIANT which
will never do anything for us.

Change-Id: I973a24d43563f289d4a2baa6e067fbba72d17ed9
Reviewed-on: http://gerrit.openafs.org/3656
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>