batman-adv: make DAT drop ARP requests targeting local clients

In the outgoing ARP request snooping routine in DAT, ARP
Request sent by local clients which are supposed to be
replied by other local clients can be silently dropped.

The destination host will reply by itself through the LAN
and therefore there is no need to involve DAT.

Reported-by: Carlos Quijano <carlos@crqgestion.es>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Tested-by: Carlos Quijano <carlos@crqgestion.es>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>