RDMA/rtrs: fix uninitialized symbol 'cnt'

RDMA/rtrs: fix uninitialized symbol 'cnt'

rtrs_clt_rdma_cq_direct returns an ninitialized value in cnt
if there is no session. This patch makes rtrs_clt_rdma_cq_direct
returns a negative value for block layer not to try again.

Fixes: 2958a995edc94 ("block/rnbd-clt: Support polling mode for IO latency optimization")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Gioh Kim <gi-oh.kim@ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20210429092741.266533-1-gi-oh.kim@ionos.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>