block: mark flush request as IDLE when it is really finished

mainline inclusion
from mainline-5.10-rc5
commit	9f16a667
category: bugfix
bugzilla: 45589
CVE: NA

-------------------------------------------------

For avoiding use-after-free on flush request, we call its .end_io() from
both timeout code path and __blk_mq_end_request().

When flush request's ref doesn't drop to zero, it is still used, we
can't mark it as IDLE, so fix it by marking IDLE when its refcount drops
to zero really.

Fixes: 65ff5cd0 ("blk-mq: mark flush request as IDLE in flush_end_io()")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Cc: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Conflicts:
        block/blk-flush.c

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Yufen Yu <yuyufen@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>