rbd: fix snapshot context reference count for discards

Discards take a reference to the snapshot context of an image when
they are created.  This reference needs to be cleaned up when the
request is done just as it is for regular writes.
Signed-off-by: NJosh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -2170,7 +2170,8 @@ static void rbd_img_request_destroy(struct kref *kref)
 		rbd_dev_parent_put(img_request->rbd_dev);
 	}
 
-	if (img_request_write_test(img_request))
+	if (img_request_write_test(img_request) ||
+		img_request_discard_test(img_request))
 		ceph_put_snap_context(img_request->snapc);
 
 	kmem_cache_free(rbd_img_request_cache, img_request);