bonding support for IPv6 transmit hashing

Currently the "bonding" driver does not support load balancing outgoing traffic in LACP mode for IPv6 traffic. IPv4 (and TCP or UDP over IPv4) are currently supported; this patch adds transmit hashing for IPv6 (and TCP or UDP over IPv6), bringing IPv6 up to par with IPv4 support in the bonding driver.

The algorithm chosen (xor'ing the bottom three quads and then xor'ing the bottom three bytes of that) was chosen after testing almost 400,000 unique IPv6 addresses harvested from server logs. This algorithm had the most even distribution for both big- and little-endian architectures while still using few instructions.

Fragmented IPv6 packets are handled the same way as fragmented IPv4 packets, ie, they are not balanced based on layer 4 information. Additionally, IPv6 packets with intermediate headers are not balanced based on layer 4 information. In practice these intermediate headers are not common and this should not cause any problems, and the alternative (a packet-parsing loop and look-up table) seemed slow and complicated for little gain.

This is an update to a prior patch I submitted. This version includes a clarified description, more thorough bounds checking, updates functions to call bond_xmit_hash_policy_l2 rather than re-implement the same logic, incorporates Jay's style suggestions, and patches against net-next. Patch has been tested and performs as expected.

John