rbd: silence bogus uninitialized use warning in rbd_acquire_lock()

  drivers/block/rbd.c: In function 'rbd_acquire_lock':
  drivers/block/rbd.c:3602:44: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Silence the warning, found it when built old kernel(3.10) with
OBS(opensuse build service).
Signed-off-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: NIlya Dryomov <idryomov@gmail.com>

drivers/block/rbd.c

@@ -3435,7 +3435,7 @@ static void rbd_acquire_lock(struct work_struct *work)
 	struct rbd_device *rbd_dev = container_of(to_delayed_work(work),
 					    struct rbd_device, lock_dwork);
 	enum rbd_lock_state lock_state;
-	int ret;
+	int ret = 0;
 
 	dout("%s rbd_dev %p\n", __func__, rbd_dev);
 again: