chown_mapped_root: fix assumption that calling uid == gid

Because if they are not, then we'll fail trying to map that gid into the
container.

The function doesn't change any gids, but lxc-usernsexec always does
setgid(0), so just map getgid() to 0 in the container.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>