nfsd: Convert nfs3xdr to use kuids and kgids

When reading uids and gids off the wire convert them to kuids and
kgids.

When putting kuids and kgids onto the wire first convert them to uids
and gids the other side will understand.

Cc: "J. Bruce Fields" <bfields@fieldses.org>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>