This patch fixes one issue reported by Kent, which can
be triggered in bcachefs over sata disk. Actually it
is a generic issue in block flush vs. blk-tag.

Cc: Christoph Hellwig <hch@infradead.org>
Reported-by: NKent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: NMing Lei <tom.leiming@gmail.com>
Signed-off-by: NJens Axboe <axboe@fb.com>