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/main.c b/fs/gfs2/main.c
index eacd78a..5b31f77 100644
--- a/fs/gfs2/main.c
+++ b/fs/gfs2/main.c
@@ -114,7 +114,7 @@
 	if (error)
 		goto fail_unregister;
 
-	error = slow_work_register_user();
+	error = slow_work_register_user(THIS_MODULE);
 	if (error)
 		goto fail_slow;
 
@@ -163,7 +163,7 @@
 	gfs2_unregister_debugfs();
 	unregister_filesystem(&gfs2_fs_type);
 	unregister_filesystem(&gfs2meta_fs_type);
-	slow_work_unregister_user();
+	slow_work_unregister_user(THIS_MODULE);
 
 	kmem_cache_destroy(gfs2_quotad_cachep);
 	kmem_cache_destroy(gfs2_rgrpd_cachep);