block: add iostat counters for flush requests

		17 - sectors discarded

		18 - time spent discarding

		Kernel 5.5+ appends two more fields for flush requests:

		19 - flush requests completed successfully

		20 - time spent flushing

		For more details refer to Documentation/admin-guide/iostats.rst

		 9 - I/Os currently in progress

		10 - time spent doing I/Os (ms)

		11 - weighted time spent doing I/Os (ms)

		12 - discards completed

		13 - discards merged

		14 - sectors discarded

		15 - time spent discarding (ms)

		16 - flush requests completed

		17 - time spent flushing (ms)

		For more details refer Documentation/admin-guide/iostats.rst

    This is the total number of milliseconds spent by all discards (as

    measured from __make_request() to end_that_request_last()).

Field 16 -- # of flush requests completed

    This is the total number of flush requests completed successfully.

    Block layer combines flush requests and executes at most one at a time.

    This counts flush requests executed by disk. Not tracked for partitions.

Field 17 -- # of milliseconds spent flushing

    This is the total number of milliseconds spent by all flush requests.

To avoid introducing performance bottlenecks, no locks are held while

modifying these counters.  This implies that minor inaccuracies may be

introduced when changes collide, so (for instance) adding up all the

discard merges  requests      number of discard I/Os merged with in-queue I/O

discard sectors sectors       number of sectors discarded

discard ticks   milliseconds  total wait time for discard requests

flush I/Os      requests      number of flush I/Os processed

flush ticks     milliseconds  total wait time for flush requests

=============== ============= =================================================

read I/Os, write I/Os, discard I/0s

These values increment when an I/O request completes.

flush I/Os

==========

These values increment when an flush I/O request completes.

Block layer combines flush requests and executes at most one at a time.

This counts flush requests executed by disk. Not tracked for partitions.

read merges, write merges, discard merges

=========================================

standard UNIX 512-byte sectors, not any device- or filesystem-specific

block size.  The counters are incremented when the I/O completes.

read ticks, write ticks, discard ticks

======================================

read ticks, write ticks, discard ticks, flush ticks

===================================================

These values count the number of milliseconds that I/O requests have

waited on this block device.  If there are multiple I/O requests waiting,

	blk_mq_add_to_requeue_list(rq, add_front, true);

}

static void blk_account_io_flush(struct request *rq)

{

	struct hd_struct *part = &rq->rq_disk->part0;

	part_stat_lock();

	part_stat_inc(part, ios[STAT_FLUSH]);

	part_stat_add(part, nsecs[STAT_FLUSH],

		      ktime_get_ns() - rq->start_time_ns);

	part_stat_unlock();

}

/**

 * blk_flush_complete_seq - complete flush sequence

 * @rq: PREFLUSH/FUA request being sequenced

	case REQ_FSEQ_DONE:

		/*

		 * @rq was previously adjusted by blk_flush_issue() for

		 * @rq was previously adjusted by blk_insert_flush() for

		 * flush sequencing and may already have gone through the

		 * flush data request completion path.  Restore @rq for

		 * normal completion and end it.

	struct blk_flush_queue *fq = blk_get_flush_queue(q, flush_rq->mq_ctx);

	struct blk_mq_hw_ctx *hctx;

	blk_account_io_flush(flush_rq);

	/* release the tag's ownership to the req cloned from */

	spin_lock_irqsave(&fq->mq_flush_lock, flags);