slow-work: use get_ref wrapper instead of directly calling get_ref

Otherwise we can get an oops if the user has no get_ref/put_ref
requirement.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/slow-work.c

@@ -637,7 +637,7 @@ int delayed_slow_work_enqueue(struct delayed_slow_work *dwork,
 			goto cancelled;
 
 		/* the timer holds a reference whilst it is pending */
-		ret = work->ops->get_ref(work);
+		ret = slow_work_get_ref(work);
 		if (ret < 0)
 			goto cant_get_ref;