Squashed revert of recent blk-mq changes

Revert "BACKPORT: blk-mq: fix is_flush_rq"

This reverts commit 09de4ca5.

Revert "BACKPORT: blk-mq: clearing flush request reference in tags->rqs[]"

This reverts commit bcf59d6f.

Revert "BACKPORT: blk-mq: grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter"

This reverts commit 66b99c5d.

Reason for revert: According to the following links, those changes are
not needed. Our kernel already has the proper version of "BACKPORT:
blk-mq: clear stale request in tags->rq[] before freeing one request pool"
and the remaining changes break compilation, so just revert them.
https://android-review.googlesource.com/c/kernel/common/+/2502118/
https://android-review.googlesource.com/c/kernel/common/+/2502120/
https://android-review.googlesource.com/c/kernel/common/+/2502121/

Change-Id: I368ab4b975089b5efc08bd7c8c19976b3934412a