namespaces: dump mount namespaces before tasks (v2)

because we want to check, that all files are reachable.
For that we need to collect all mounts from all namespaces.

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