tcp: md5: increment sk_drops on syn_recv state

TCP MD5 mismatches do increment sk_drops counter in all states but
SYN_RECV.

This is very unlikely to happen in the real world, but worth adding
to help diagnostics.

We increase the parent (listener) sk_drops.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -1602,6 +1602,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
 
 		sk = req->rsk_listener;
 		if (unlikely(tcp_v4_inbound_md5_hash(sk, skb))) {
+			sk_drops_add(sk, skb);
 			reqsk_put(req);
 			goto discard_it;
 		}

net/ipv6/tcp_ipv6.c

@@ -1409,6 +1409,7 @@ static int tcp_v6_rcv(struct sk_buff *skb)
 		sk = req->rsk_listener;
 		tcp_v6_fill_cb(skb, hdr, th);
 		if (tcp_v6_inbound_md5_hash(sk, skb)) {
+			sk_drops_add(sk, skb);
 			reqsk_put(req);
 			goto discard_it;
 		}