nfsd: add a per-net-namespace struct for nfsd

Eventually, we'll need this when nfsd gets containerized fully. For
now, create a struct on a per-net-namespace basis that will just hold
a pointer to the cld_net structure. That struct will hold all of the
per-net data that we need for the cld tracker.

Eventually we can add other pernet objects to struct nfsd_net.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>