btrfs: index check-integrity state hash by a dev_t

We won't have the struct block_device available in the bio soon, so switch
to the numerical dev_t instead of the block_device pointer for looking up
the check-integrity state.

Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>