nfsd: Convert nfsxdr to use kuids and kgids

When reading uids and gids off the wire convert them to
kuids and kgids.  If the conversion results in an invalid
result don't set the ATTR_UID or ATTR_GID.

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>