userns: Handle -1 in k[ug]id_has_mapping when !CONFIG_USER_NS

Refuse to admit any user namespace has a mapping of the INVALID_UID
and the INVALID_GID when !CONFIG_USER_NS.

Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>