SLOW_WORK: Wait for outstanding work items belonging to a module to clear

Wait for outstanding slow work items belonging to a module to clear when
unregistering that module as a user of the facility.  This prevents the put_ref
code of a work item from being taken away before it returns.

Signed-off-by: David Howells <dhowells@redhat.com>
diff --git a/fs/gfs2/recovery.c b/fs/gfs2/recovery.c
index 59d2695..b2bb779 100644
--- a/fs/gfs2/recovery.c
+++ b/fs/gfs2/recovery.c
@@ -593,6 +593,7 @@
 }
 
 struct slow_work_ops gfs2_recover_ops = {
+	.owner	 = THIS_MODULE,
 	.get_ref = gfs2_recover_get_ref,
 	.put_ref = gfs2_recover_put_ref,
 	.execute = gfs2_recover_work,