net: rtnetlink: address is mandatory for rtnl_fdb_get

We must have an address to lookup otherwise we'll derefence a null
pointer in the ndo_fdb_get callbacks.

CC: Roopa Prabhu <roopa@cumulusnetworks.com>
CC: David Ahern <dsa@cumulusnetworks.com>
Reported-by: syzbot+017b1f61c82a1c3e7efd@syzkaller.appspotmail.com
Fixes: 5b2f94b ("net: rtnetlink: support for fdb get")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -4104,6 +4104,11 @@ static int rtnl_fdb_get(struct sk_buff *in_skb, struct nlmsghdr *nlh,
 	if (err < 0)
 		return err;
 
+	if (!addr) {
+		NL_SET_ERR_MSG(extack, "Missing lookup address for fdb get request");
+		return -EINVAL;
+	}
+
 	if (brport_idx) {
 		dev = __dev_get_by_index(net, brport_idx);
 		if (!dev) {