batman-adv: Ignore invalid batadv_iv_gw during netlink send

The function batadv_iv_gw_dump stops the processing loop when
batadv_iv_gw_dump_entry returns a non-0 return code. This should only
happen when the buffer is full. Otherwise, an empty message may be
returned by batadv_gw_dump. This empty message will then stop the netlink
dumping of gateway entries. At worst, not a single entry is returned to
userspace even when plenty of possible gateways exist.

Fixes: efb766af ("batman-adv: add B.A.T.M.A.N. IV bat_gw_dump implementations")
Signed-off-by: Sven Eckelmann <sven.eckelmann@openmesh.com>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>