tcp: fastopen: avoid negative sk_forward_alloc

When DATA and/or FIN are carried in a SYN/ACK message or SYN message,
we append an skb in socket receive queue, but we forget to call
sk_forced_mem_schedule().

Effect is that the socket has a negative sk->sk_forward_alloc as long as
the message is not read by the application.

Josh Hunt fixed a similar issue in commit d22e1537 ("tcp: fix tcp
fin memory accounting")

Fixes: 168a8f58 ("tcp: TCP Fast Open Server - main code path")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reviewed-by: NJosh Hunt <johunt@akamai.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_fastopen.c

@@ -150,6 +150,7 @@ void tcp_fastopen_add_skb(struct sock *sk, struct sk_buff *skb)
 	tp->segs_in = 0;
 	tcp_segs_in(tp, skb);
 	__skb_pull(skb, tcp_hdrlen(skb));
+	sk_forced_mem_schedule(sk, skb->truesize);
 	skb_set_owner_r(skb, sk);
 
 	TCP_SKB_CB(skb)->seq++;