NFSD: allow callers to pass in scope string to nfsd_svc

Currently admins set this by using unshare to create a new uts
namespace, and then resetting the hostname. With the new netlink
interface we can just pass this in directly. Prepare nfsd_svc for
this change.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>