Each mountpoint belongs to a mount namespace, so we need to
find a root of the mount namespace and open mountpoint
ralative to this root.

The same logic is used in get_mark_path().
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>