blk-mq: cleanup __blk_mq_sched_dispatch_requests

__blk_mq_sched_dispatch_requests currently has duplicated logic
for the cases where requests are on the hctx dispatch list or not.
Merge the two with a new need_dispatch variable and remove a few
pointless local variables.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20230413060651.694656-2-hch@lst.de


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