batman-adv: allow updating DAT entry timeouts on incoming ARP Replies

Currently incoming ARP Replies, for example via a DHT-PUT message, do
not update the timeout for an already existing DAT entry. These ARP
Replies are dropped instead.

This however defeats the purpose of the DHCPACK snooping, for instance.
Right now, a DAT entry in the DHT will be purged every five minutes,
likely leading to a mesh-wide ARP Request broadcast after this timeout.
Which then recreates the entry. The idea of the DHCPACK snooping is to
be able to update an entry before a timeout happens, to avoid ARP Request
flooding.

This patch fixes this issue by updating a DAT entry on incoming
ARP Replies even if a matching DAT entry already exists. While still
filtering the ARP Reply towards the soft-interface, to avoid duplicate
messages on the client device side.

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