[GFS2] don't try to lockfs after shutdown

If an fs has already been shut down, a lockfs callback should do nothing.
An fs that's been shut down can't acquire locks or do anything with
respect to the cluster.

Also, remove FIXME comment in withdraw function.  The missing bits of the
withdraw procedure are now all done by user space.

Signed-off-by: David Teigland <teigland@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>