NFS: Convert NFSv3 to use the container user namespace

When mapping NFS identities, we want to substitute for the uids and
gids on the wire as we would for the AUTH_UNIX creds.

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