mounts: Make sure all namespaces has the same root source

Currently we support nested mount namespaces, only if all of them have
the same path to the root.

crtools doesn't mount root, so it should be bind-mounted for all namespaces.

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