nvmet: use unlikely for req status check

This patch adds unlikely in the nvmet request completion path for the
status check in the low level function __nvmet_req_complete.
This is helpful in the scenario where host and target connection is
working smoothly.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>