nvme: implement ->get_unique_id

Implement the get_unique_id method to allow pNFS SCSI layout access to
NVMe namespaces.

This is the server side implementation of RFC 9561 "Using the Parallel
NFS (pNFS) SCSI Layout to Access Non-Volatile Memory Express (NVMe)
Storage Devices".

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>