bonding: use l4 hash if available

If skb carries a l4 hash, no need to perform a flow dissection.

Performance is slightly better :

lpaa5:~# ./super_netperf 200 -H lpaa6 -t TCP_RR -l 100
2.39012e+06
lpaa5:~# ./super_netperf 200 -H lpaa6 -t TCP_RR -l 100
2.39393e+06
lpaa5:~# ./super_netperf 200 -H lpaa6 -t TCP_RR -l 100
2.39988e+06

After patch :

lpaa5:~# ./super_netperf 200 -H lpaa6 -t TCP_RR -l 100
2.43579e+06
lpaa5:~# ./super_netperf 200 -H lpaa6 -t TCP_RR -l 100
2.44304e+06
lpaa5:~# ./super_netperf 200 -H lpaa6 -t TCP_RR -l 100
2.44312e+06

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <tom@herbertland.com>
Cc: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -3136,6 +3136,10 @@ u32 bond_xmit_hash(struct bonding *bond, struct sk_buff *skb)
 	struct flow_keys flow;
 	u32 hash;
 
+	if (bond->params.xmit_policy == BOND_XMIT_POLICY_ENCAP34 &&
+	    skb->l4_hash)
+		return skb->hash;
+
 	if (bond->params.xmit_policy == BOND_XMIT_POLICY_LAYER2 ||
 	    !bond_flow_dissect(bond, skb, &flow))
 		return bond_eth_hash(skb);