mount: bind-mount root into itself if processes are restored in userns

When we create a new mntns in a userns, all inhereted mounts are marked
as locked. pivot_root() returns EINVAL if a new root is locked.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>