NVMe: blk_mq_alloc_request() returns error pointers

We recently converted this to blk_mq but the error checks have to be
updated to check for IS_ERR() instead of NULL.

Fixes: a4aea562 ('NVMe: Convert to blk-mq')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@fb.com>