net: Improve simple_tx_hash().

Based upon feedback from Eric Dumazet and Andi Kleen.

Cure several deficiencies in simple_tx_hash() by using
jhash + reciprocol multiply.

1) Eliminates expensive modulus operation.

2) Makes hash less attackable by using random seed.

3) Eliminates endianness hash distribution issues.

Signed-off-by: David S. Miller <davem@davemloft.net>