batman-adv: tvlv: prepare for tvlv enabled multicast packet type

Prepare TVLV infrastructure for more packet types, in particular the
upcoming batman-adv multicast packet type.

For that swap the OGM vs. unicast-tvlv packet boolean indicator to an
explicit unsigned integer packet type variable. And provide the skb
to a call to batadv_tvlv_containers_process(), as later the multicast
packet's TVLV handler will need to have access not only to the TVLV but
the full skb for forwarding. Forwarding will be invoked from the
multicast packet's TVLVs' contents later.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>