sd: implement ->get_unique_id

Add the method to query for a uniqueue ID of a given type by looking
it up in the cached device identification VPD page.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20211021060607.264371-3-hch@lst.de


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