[PATCH v3 10/10] blk-mq: support per-distpatch_queue flush machinery

This patch supports to run one single flush machinery for each blk-mq dispatch queue, so that: - current init_request and exit_request callbacks can cover flush request too, then the buggy copying way of initializing flush request's pdu can be fixed - flushing performance gets improved in case

[PATCH v3 10/10] blk-mq: support per-distpatch_queue flush machinery

This patch supports to run one single flush machinery for each blk-mq dispatch queue, so that: - current init_request and exit_request callbacks can cover flush request too, then the buggy copying way of initializing flush request's pdu can be fixed - flushing performance gets improved in case