gfs2: Fix "ignore unlock failures after withdraw"

gfs2: Fix "ignore unlock failures after withdraw"

Commit 3e11e53041502 tries to suppress dlm_lock() lock conversion errors
that occur when the lockspace has already been released.

It does that by setting and checking the SDF_SKIP_DLM_UNLOCK flag.  This
conflicts with the intended meaning of the SDF_SKIP_DLM_UNLOCK flag, so
check whether the lockspace is still allocated instead.

(Given the current DLM API, checking for this kind of error after the
fact seems easier that than to make sure that the lockspace is still
allocated before calling dlm_lock().  Changing the DLM API so that users
maintain the lockspace references themselves would be an option.)

Fixes: 3e11e53041502 ("GFS2: ignore unlock failures after withdraw")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>