net: Recompute sk_txhash on negative routing advice

When a connection is failing a transport protocol calls
dst_negative_advice to try to get a better route. This patch includes
changing the sk_txhash in that function. This provides a rudimentary
method to try to find a different path in the network since sk_txhash
affects ECMP on the local host and through the network (via flow labels
or UDP source port in encapsulation).
Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -1695,6 +1695,12 @@ static inline void sk_set_txhash(struct sock *sk)
 		sk->sk_txhash = 1;
 }
 
+static inline void sk_rethink_txhash(struct sock *sk)
+{
+	if (sk->sk_txhash)
+		sk_set_txhash(sk);
+}
+
 static inline struct dst_entry *
 __sk_dst_get(struct sock *sk)
 {
@@ -1719,6 +1725,8 @@ static inline void dst_negative_advice(struct sock *sk)
 {
 	struct dst_entry *ndst, *dst = __sk_dst_get(sk);
 
+	sk_rethink_txhash(sk);
+
 	if (dst && dst->ops->negative_advice) {
 		ndst = dst->ops->negative_advice(dst);