block: add rationale for not using blk_mq_plug() when applicable

There are two places in the block layer at the moment where
blk_mq_plug() helper could be used instead of directly accessing the
plug from struct current. In both these cases, directly accessing the plug
should not have any consequences for zoned devices.

Make the intent explicit by adding comments instead of introducing unwanted
checks with blk_mq_plug() helper.[1]

[1] https://lore.kernel.org/linux-block/f6e54907-1035-2b2c-6387-ed178be05ccb@kernel.dk/

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Suggested-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/20220929144141.140077-1-p.raghav@samsung.com
[axboe: fixup multi-line comment style]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -850,6 +850,12 @@ int bio_poll(struct bio *bio, struct io_comp_batch *iob, unsigned int flags)
 	    !test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
 		return 0;
 
+	/*
+	 * As the requests that require a zone lock are not plugged in the
+	 * first place, directly accessing the plug instead of using
+	 * blk_mq_plug() should not have any consequences during flushing for
+	 * zoned devices.
+	 */
 	blk_flush_plug(current->plug, false);
 
 	if (bio_queue_enter(bio))

block/blk-mq.c

@@ -1215,6 +1215,12 @@ void blk_execute_rq_nowait(struct request *rq, bool at_head)
 	WARN_ON(!blk_rq_is_passthrough(rq));
 
 	blk_account_io_start(rq);
+
+	/*
+	 * As plugging can be enabled for passthrough requests on a zoned
+	 * device, directly accessing the plug instead of using blk_mq_plug()
+	 * should not have any consequences.
+	 */
 	if (current->plug)
 		blk_add_rq_to_plug(current->plug, rq);
 	else