Riya Khanna reported that with a ramfs rootfs the mount to make
/ rprivate was returning -EFAULT.  NULL was being passed as the
mount target.  Pass "/" instead.
Reported-by: riya khanna <riyakhanna1983@gmail.com&gt;>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>