tcp: clear hints to avoid a stale one (nfs only affected?)

Eric Dumazet mentioned in a context of another problem:

"Well, it seems NFS reuses its socket, so maybe we miss some
cleaning as spotted in this old patch"

I've not check under which conditions that actually happens but
if true, we need to make sure we don't accidently leave stale
hints behind when the write queue had to be purged (whether reusing
with NFS can actually happen if purging took place is something I'm
not sure of).

...At least it compiles.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1229,6 +1229,7 @@ static inline void tcp_write_queue_purge(struct sock *sk)
 	while ((skb = __skb_dequeue(&sk->sk_write_queue)) != NULL)
 		sk_wmem_free_skb(sk, skb);
 	sk_mem_reclaim(sk);
+	tcp_clear_all_retrans_hints(tcp_sk(sk));
 }
 
 static inline struct sk_buff *tcp_write_queue_head(struct sock *sk)