blk-mq: mark flush request as IDLE in flush_end_io()

mainline inclusion
from mainline-5.10-rc2
commit	65ff5cd0
category: bugfix
bugzilla: 45589
CVE: NA

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

Mark flush request as IDLE in its .end_io(), aligning it with how normal
requests behave. The flush request stays in in-flight tags if we're not
using an IO scheduler, so we need to change its state into IDLE.
Otherwise, we will hang in blk_mq_tagset_wait_completed_request() during
error recovery because flush the request state is kept as COMPLETED.

Reported-by: Yi Zhang <yi.zhang@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Cc: Chao Leng <lengchao@huawei.com>
Cc: Sagi Grimberg <sagi@grimberg.me>
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>