This adds support to obtain a device's unique id through dm, similar to the
existing ioctl and persistent resevation handling. We limit this to
single-target devices.
This enables knfsd to export pNFS SCSI luns that have been exported from
multipath devices.
Signed-off-by: Benjamin Coddington <bcodding@redhat.com> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com> Reviewed-by: Christoph Hellwig <hch@lst.de>