batman-adv: network coding - save overheard and tx packets for decoding

To be able to decode a network coded packet, a node must already know
one of the two coded packets. This is done by buffering skbs before
transmission and buffering packets sniffed with promiscuous mode from
other hosts.

Packets are kept in a buffer similar to the one with forward-skbs: A
hash table, where each entry, which corresponds to a src-dst pair, has a
linked list packets.

Signed-off-by: Martin Hundebøll <martin@hundeboll.net>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>