block: check if a profile is actually registered in blk_integrity_unregister

While clearing the profile itself is harmless, we really should not clear
the stable writes flag if it wasn't set due to a registered integrity
profile.

Reported-by: Lihong Kou <koulihong@huawei.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Link: https://lore.kernel.org/r/20210914070657.87677-2-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>