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/fscache/operation.c b/fs/fscache/operation.c
index e7f8d53..f1a2857 100644
--- a/fs/fscache/operation.c
+++ b/fs/fscache/operation.c
@@ -453,6 +453,7 @@
 }
 
 const struct slow_work_ops fscache_op_slow_work_ops = {
+	.owner		= THIS_MODULE,
 	.get_ref	= fscache_op_get_ref,
 	.put_ref	= fscache_op_put_ref,
 	.execute	= fscache_op_execute,