Only do the funky chroot_into_slave if / is in fact the rootfs.
Rootfs is a special blacklisted case for pivot_root.

If / is not rootfs but is shared, just mount / rslave.  We're
already in our own namespace.

This appears to solve the extra /proc/$$/mount entries in
containers and the host directories in lxc-attach which have
been plagueing at least fedora and arch.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>