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>