bonding: enhance L2 hash helper with packet type

Current L2 hash helper calculates destination eth addr and
source ether addr as L2 hash factors.  This patch is adding
packet type ID field into L2 hash factors.  While one of
BOND_XMIT_POLICY_LAYER2 or BOND_XMIT_POLICY_{LAYER|ENCAP}23
is applied, for the 2nd level hash, enhanced hash method can
help to distribute different types of packets like IPv4/IPv6
packets to different slave devices.

CC: Jay Vosburgh <j.vosburgh@gmail.com>
CC: Veaceslav Falico <vfalico@gmail.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: David S. Miller <davem@davemloft.net>
CC: Pan Jiafei <Jiafei.Pan@freescale.com>

Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jianhua Xie <jianhua.xie@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>