BACKPORT: blk-mq: fix is_flush_rq

is_flush_rq() is called from bt_iter()/bt_tags_iter(), and runs the
following check:

	hctx->fq->flush_rq == req

but the passed hctx from bt_iter()/bt_tags_iter() may be NULL because:

1) memory re-order in blk_mq_rq_ctx_init():

	rq->mq_hctx = data->hctx;
	...
	refcount_set(&rq->ref, 1);

OR

2) tag re-use and ->rqs[] isn't updated with new request.

Fix the issue by re-writing is_flush_rq() as:

	return rq->end_io == flush_end_io;

which turns out simpler to follow and immune to data race since we have
ordered WRITE rq->end_io and refcount_set(&rq->ref, 1).

Fixes: 2e315dc07df0 ("blk-mq: grab rq->refcount before calling ->fn in
    blk_mq_tagset_busy_iter")
Cc: "Blank-Burian, Markus, Dr." <blankburian@uni-muenster.de>
Cc: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>

Bug: 197804811
Change-Id: Iba551d228e5cd96749c13f4c19e13687b0e89196
(cherry picked from commit a9ed27a764156929efe714033edb3e9023c5f321)
[Pradeep: Resolved conflicts in block/blk.h]
Signed-off-by: Pradeep P V K <pragalla@codeaurora.org>