blk-mq: allow hardware queue to get more tag while sharing a tag set

hulk inclusion
category: performance
bugzilla: https://gitee.com/openeuler/kernel/issues/I4S8DW



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When sharing a tag set, if most disks are issuing small amount of IO, and
only a few is issuing a large amount of IO. Current approach is to limit
the max amount of tags a disk can get equally to the average of total
tags. Thus the few heavy load disk can't get enough tags while many tags
are still free in the tag set.

We add 'pending_queues' in blk_mq_tag_set to count how many queues can't
get driver tag. Thus if this value is zero, there is no need to limit
the max number of available tags.

On the other hand, if a queue doesn't issue IO, the 'active_queues' will
not be decreased in a period of time(request timeout), thus a lot of tags
will not be available because max number of available tags is set to
max(total tags / active_queues, 4). Thus we decreased it when
'nr_active' is 0.

This functionality is enabled by default, to disable it, add
"blk_mq.unfair_dtag=0" to boot cmd.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>