gfs2: Allow gfs2_xattr_set to be called with the glock held

On the following call path:

  gfs2_setattr -> setattr_prepare -> ... ->
    cap_inode_killpriv -> ... ->
      gfs2_xattr_set

the glock is locked in gfs2_setattr, so check for recursive locking in
gfs2_xattr_set as gfs2_xattr_get already does.  While at it, get rid of
need_unlock in gfs2_xattr_get.

Fixes xfstest generic/093.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Acked-by: Abhijith Das <adas@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>