nbd: Fix error handling

'blk_mq_alloc_request()' returns an error pointer in case of error, not
NULL. So test it with IS_ERR.

Fixes: 	fd8383fd ("nbd: convert to blkmq")

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jens Axboe <axboe@fb.com>