ns: Introduce ns walking helper

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
net: Use ns walking helper

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
mnt: Clean up namespaces walking code

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
mnt: Use ns walking helper

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>