net: allow dumping of gre0 and gretap0

These devices are injected into every network namespace when the ip_gre
module is loaded. They cannot be configured as usable tunnels, so let's
just ignore them.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>