Currently collect_shared is executed for each mount namespace separately,
so we miss the dependencies between namespaces.

For example, we have two namespaces, the first one contains a tmpfs mount
and the second one contains a non-root bind-mount of this tmpfs. Without
this patch this example can't be dumped.

On restore mnt_build_tree() is called for all namespaces at once, thus
this bug doesn't exist there.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>