chown_mapped_rootid: don't double-map root id

If the user maps container root to his host uid, chown_mapped_rootid
tries to make the same mapping twice and gets -EINVAL.

Reported-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>