cfg80211: allow vendor dumpit to terminate by returning 0

nl80211 vendor netlink dumpit, like netlink_callback->dump, should
signal successful completion by returning 0.

Currently, that will just cause dumpit to be called again, possibly
many times until an error occurs.  Since skb->len is never going to be
0 by the time dumpit is called, the only way for dumpit to signal
completion is by returning an error.  If it returns a positive value,
the current message is cancelled, but that positive value is returned
and nl80211_vendor_cmd_dump gets called again.

Fix that by passing a return value of 0 through.
Signed-off-by: Julian Squires <julian@cipht.net>
Link: https://lore.kernel.org/r/20200720145033.401307-1-julian@cipht.net
[reword commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -13479,7 +13479,7 @@ static int nl80211_vendor_cmd_dump(struct sk_buff *skb,
 		if (err == -ENOBUFS || err == -ENOENT) {
 			genlmsg_cancel(skb, hdr);
 			break;
-		} else if (err) {
+		} else if (err <= 0) {
 			genlmsg_cancel(skb, hdr);
 			goto out;
 		}