block: add a ->get_unique_id method

Add a method to query unique IDs from block devices.  It will be used to
remove code that deeply pokes into SCSI internals in the NFS server.
The implementation in the sd driver itself is also much nicer as it can
use the cached VPD page instead of always sending a command as the
current NFS code does.

For now the interface is kept very minimal but could be easily
extended when other users like a block-layer sysfs interface for
uniquue IDs shows up.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20211021060607.264371-2-hch@lst.de


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