tipc: fix use-after-free in tipc_disc_rcv()

In the function 'tipc_disc_rcv()', the 'msg_peer_net_hash()' is called
to read the header data field but after the message skb has been freed,
that might result in a garbage value...

This commit fixes it by defining a new local variable to store the data
first, just like the other header fields' handling.

Fixes: f73b12812a3d ("tipc: improve throughput between nodes in netns")
Acked-by: Jon Maloy <[e-mail]>
Signed-off-by: Tuong Lien <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/tipc/discover.c

···
 {
 	struct tipc_net *tn = tipc_net(net);
 	struct tipc_msg *hdr = buf_msg(skb);
+	u32 pnet_hash = msg_peer_net_hash(hdr);
 	u16 caps = msg_node_capabilities(hdr);
 	bool legacy = tn->legacy_addr_format;
 	u32 sugg = msg_sugg_node_addr(hdr);
···
 		return;
 	if (!tipc_in_scope(legacy, b->domain, src))
 		return;
-	tipc_node_check_dest(net, src, peer_id, b, caps, signature,
-			     msg_peer_net_hash(hdr), &maddr, &respond,
-			     &dupl_addr);
+	tipc_node_check_dest(net, src, peer_id, b, caps, signature, pnet_hash,
+			     &maddr, &respond, &dupl_addr);
 	if (dupl_addr)
 		disc_dupl_alert(b, src, &maddr);
 	if (!respond)