sr: simplify sr_block_revalidate_disk

Both callers have a valid CD struture available, so rely on that instead
of getting another reference.  Also move the function to avoid a forward
declaration.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>