batman-adv: Add wrapper to look up neighbor and send skb

By adding batadv_send_skb_to_orig() in send.c, we can remove duplicate
code that looks up the next hop and then calls batadv_send_skb_packet().

Furthermore, this prepares the upcoming new implementation of
fragmentation, which requires the next hop to route packets.

Please note that this doesn't entirely remove the next-hop lookup in
routing.c and unicast.c, since it is used by the current fragmentation
code.

Also note that the next-hop info is removed from debug messages in
translation-table.c, since it is looked up elsewhere.

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>