batman-adv: Map VID 0 to untagged TT VLAN

VID 0 is not a valid VLAN according to "802.1Q-2011" "Table 9-2—Reserved
VID values". It is only used to indicate "priority tag" frames which only
contain priority information and no VID.

The 8021q is also redirecting the priority tagged frames to the underlying
interface since commit ad1afb00 ("vlan_dev: VLAN 0 should be treated as
"no vlan tag" (802.1p packet)"). But at the same time, it automatically
adds the VID 0 to all devices to ensure that VID 0 is in the allowed list
of the HW filter. This resulted in a VLAN 0 which was always announced in
OGM messages.

batman-adv should therefore not create a new batadv_softif_vlan for VID 0
and handle all VID 0 related frames using the "untagged" global/local
translation tables.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Antonio Quartulli <antonio@mandelbit.com>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>