GFS2: Reinstate withdraw ack system

This patch reinstates the ack system which withdraw should be using. It
appears to have been accidentally forgotten when the lock module was
merged into GFS2, due to two different sysfs files having the same name.

Reported-by: David Teigland <teigland@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
diff --git a/fs/gfs2/util.c b/fs/gfs2/util.c
index f00d7c5..6402fb6 100644
--- a/fs/gfs2/util.c
+++ b/fs/gfs2/util.c
@@ -54,6 +54,9 @@
 
 		kobject_uevent(&sdp->sd_kobj, KOBJ_OFFLINE);
 
+		if (!strcmp(sdp->sd_lockstruct.ls_ops->lm_proto_name, "lock_dlm"))
+			wait_for_completion(&sdp->sd_wdack);
+
 		if (lm->lm_unmount) {
 			fs_err(sdp, "telling LM to unmount\n");
 			lm->lm_unmount(sdp);