batman-adv: Fix netlink dumping of BLA backbones

The function batadv_bla_backbone_dump_bucket must be able to handle
non-complete dumps of a single bucket. It tries to do that by saving the
latest dumped index in *idx_skip to inform the caller about the current
state.

But the caller only assumes that buckets were not completely dumped when
the return code is non-zero. This function must therefore also return a
non-zero index when the dumping of an entry failed. Otherwise the caller
will just skip all remaining buckets.

And the function must also reset *idx_skip back to zero when it finished a
bucket. Otherwise it will skip the same number of entries in the next
bucket as the previous one had.

Fixes: ea4152e11716 ("batman-adv: add backbone table netlink support")
Reported-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/bridge_loop_avoidance.c

@@ -2394,22 +2394,25 @@ batadv_bla_backbone_dump_bucket(struct sk_buff *msg, u32 portid, u32 seq,
 {
 	struct batadv_bla_backbone_gw *backbone_gw;
 	int idx = 0;
+	int ret = 0;
 
 	rcu_read_lock();
 	hlist_for_each_entry_rcu(backbone_gw, head, hash_entry) {
 		if (idx++ < *idx_skip)
 			continue;
-		if (batadv_bla_backbone_dump_entry(msg, portid, seq,
-						   primary_if, backbone_gw)) {
+
+		ret = batadv_bla_backbone_dump_entry(msg, portid, seq,
+						     primary_if, backbone_gw);
+		if (ret) {
 			*idx_skip = idx - 1;
 			goto unlock;
 		}
 	}
 
-	*idx_skip = idx;
+	*idx_skip = 0;
 unlock:
 	rcu_read_unlock();
-	return 0;
+	return ret;
 }
 
 /**