Fix to work with unprivileged containers on recent kernel

Change chown_mapped_root() to map in both the root uid and gid, not
just the uid, so as to work with unprivileged containers on recent
kernel.

Signed-off-by: TAMUKI Shoichi <tamuki@linet.gr.jp>
Signed-off-by: KATOH Yasufumi <karma@jazz.email.ne.jp>