nfsd/blocklayout: use ->get_unique_id instead of sending SCSI commands

Call the ->get_unique_id method to query the SCSI identifiers.  This can
use the cached VPD page in the sd driver instead of sending a command
on every LAYOUTGET.  It will also allow to support NVMe based volumes
if the draft for that ever takes off.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: J. Bruce Fields <bfields@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/20211021060607.264371-4-hch@lst.de


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