chown_mapped_root: don't try chgrp if we don't own the file

chown_mapped_root: don't try chgrp if we don't own the file

New kernels require that to have privilege over a file, your
userns must have the old and new groups mapped into your userns.
So if a file is owned by our uid but another groupid, then we
have to chgrp the file to our primary group before we can try
(in a new user namespace) to chgrp the file to a group id in the
namespace.

But in some cases (when cloning) the file may already be mapped
into the container.  Now we cannot chgrp the file to our own
primary group - and we don't have to.

So detect that case.  Only try to chgrp the file to our primary
group if the file is owned by our euid (i.e. not by the container)
and the owning group is not already mapped into the container by
default.

With this patch, I'm again able to both create and clone containers
with no errors again.

Reported-by: S.Çağlar Onur <caglar@10ur.org>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>