batman-adv: Fix rx packet/bytes stats on local ARP reply

The stats are generated by batadv_interface_stats and must not be stored
directly in the net_device stats member variable. The batadv_priv
bat_counters information is assembled when ndo_get_stats is called. The
stats previously stored in net_device::stats is then overwritten.

The batman-adv counters must therefore be increased when an ARP packet is
answered locally via the distributed arp table.

Fixes: c384ea3e ("batman-adv: Distributed ARP Table - add snooping functions for ARP messages")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>