bonding: discard lowest hash bit for 802.3ad layer3+4

After commit 07f4c900 ("tcp/dccp: try to not exhaust ip_local_port_range
in connect()"), we will try to use even ports for connect(). Then if an
application (seen clearly with iperf) opens multiple streams to the same
destination IP and port, each stream will be given an even source port.

So the bonding driver's simple xmit_hash_policy based on layer3+4 addressing
will always hash all these streams to the same interface. And the total
throughput will limited to a single slave.

Change the tcp code will impact the whole tcp behavior, only for bonding
usage. Paolo Abeni suggested fix this by changing the bonding code only,
which should be more reasonable, and less impact.

Fix this by discarding the lowest hash bit because it contains little entropy.
After the fix we can re-balance between slaves.
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NHangbin Liu <liuhangbin@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -3253,7 +3253,7 @@ u32 bond_xmit_hash(struct bonding *bond, struct sk_buff *skb)
 	hash ^= (hash >> 16);
 	hash ^= (hash >> 8);
 
-	return hash;
+	return hash >> 1;
 }
 
 /*-------------------------- Device entry points ----------------------------*/