NFS: Implement NFSv4.2's OFFLOAD_STATUS operation

Enable the Linux NFS client to observe the progress of an offloaded
asynchronous COPY operation. This new operation will be put to use
in a subsequent patch.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Link: https://lore.kernel.org/r/20250113153235.48706-14-cel@kernel.org


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