blk-mq: handle NULL req return from blk_map_request in single queue mode

blk-mq: handle NULL req return from blk_map_request in single queue mode

blk_mq_map_request() can return NULL if we fail entering the queue
(dying, or removed), in which case it has already ended IO on the
bio. So nothing more to do, except just return.

Signed-off-by: Jens Axboe <axboe@fb.com>