Current aufs supports FS_USERNS_MOUNT by using allow_userns module
parameter. It allows root in userns to mount aufs.

This patch allows an unprivileged container to use aufs. The value of
xino option is changed to /dev/shm/aufs.xino that an unpriv user can
write.
Signed-off-by: KATOH Yasufumi <karma@jazz.email.ne.jp>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>