'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: NChristophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: NJens Axboe <axboe@fb.com>