create rslave chroot any time pivot_root fails with -EINVAL

Note we still need the function to detect a MS_SHARED /, bc
if we detect that then we need to umount the chroot tmpfs
mount which lxc-start created, whose mount was forwarded to
the host mounts namespace.

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