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: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>