batman-adv: linearise the tt_response skb only if needed

The TT_RESPONSE skb has to be linearised only if the node plans to access the
packet payload (so only if the message is directed to that node). In all the
other cases the node can avoid this memory operation

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>