tcp: increment sk_drops for dropped rx packets

Now ss can report sk_drops, we can instruct TCP to increment
this per socket counter when it drops an incoming frame, to refine
monitoring and debugging.

Following patch takes care of listeners drops.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index f0422e7..5fa8fea 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1484,6 +1484,7 @@
 	return 0;
 
 discard_and_relse:
+	sk_drops_add(sk, skb);
 	if (refcounted)
 		sock_put(sk);
 	goto discard_it;