rbd: silence bogus -Wmaybe-uninitialized warning

drivers/block/rbd.c: In function ‘rbd_watch_cb’:
drivers/block/rbd.c:3690:5: error: ‘struct_v’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/block/rbd.c:3759:5: note: ‘struct_v’ was declared here

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

@@ -3756,7 +3756,7 @@ static void rbd_watch_cb(void *arg, u64 notify_id, u64 cookie,
 	struct rbd_device *rbd_dev = arg;
 	void *p = data;
 	void *const end = p + data_len;
-	u8 struct_v;
+	u8 struct_v = 0;
 	u32 len;
 	u32 notify_op;
 	int ret;