block/mq-deadline: Reserve 25% of scheduler tags for synchronous requests

For interactive workloads it is important that synchronous requests are
not delayed. Hence reserve 25% of scheduler tags for synchronous requests.
This patch still allows asynchronous requests to fill the hardware queues
since blk_mq_init_sched() makes sure that the number of scheduler requests
is the double of the hardware queue depth. From blk_mq_init_sched():

	q->nr_requests = 2 * min_t(unsigned int, q->tag_set->queue_depth,
				   BLKDEV_MAX_RQ);

Cc: Damien Le Moal <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Ming Lei <[email protected]>
Cc: Johannes Thumshirn <[email protected]>
Cc: Himanshu Madhani <[email protected]>
Signed-off-by: Bart Van Assche <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

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