Currently we marks all mounts as private before restoring mntns. We do
these to avoid problem with pivot_root.
It's wrong, because the root mount can be slave for an external shared
group. The root mount is not mounted by CRIU, so here is nothing wrong.

Now look at the pivot_root code in kernel
if (IS_MNT_SHARED(old_mnt) ||
	IS_MNT_SHARED(new_mnt->mnt_parent) ||
	IS_MNT_SHARED(root_mnt->mnt_parent))
	goto out4;

So we don't need to change options for all mounts. We need to remount
/ and the parent of the new root. It's safe, because we already in another
mntns.

v2: simplify code
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>