block: null_blk: cleanup null_queue_rq()

Use a local struct request pointer variable to avoid having to
dereference struct blk_mq_queue_data multiple times. While at it, also
fix the function argument indentation and remove a useless "else" after
a return.

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Pankaj Raghav <p.raghav@samsung.com>
Link: https://lore.kernel.org/r/20230314041106.19173-2-damien.lemoal@opensource.wdc.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>