NFS: Add implid to sysfs

The Linux NFS server added support for returning this information during
an EXCHANGE_ID in Linux v6.13. This is something and admin might want to
query, so let's add it to sysfs.

Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>
Link: https://lore.kernel.org/r/20250207204225.594002-2-anna@kernel.org


Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>