nvmet-tcp: fix uninitialized variable access

If we end up in nvmet_tcp_try_recv_one with a bogus state
queue receive state we will access result which is uninitialized.

Initialize restult to 0 which will be considered as if no data
was received by the tcp socket.

Fixes: 872d26a3 ("nvmet-tcp: add NVMe over TCP target driver")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>