datapath: Consolidate checksum compatibility code.

datapath: Consolidate checksum compatibility code.

Checksum offloading has changed quite a bit across different kernel
and Xen versions.  Since it is part of the skb data structure it is
unfortunately difficult to separate out into compatibility code.
This consolidates all of the checksum code in one place which makes
it easier read and remove as we prepare for upstreaming.  On newer
kernels it also puts everything in inline functions, eliminating the
need to run through the compat code or make extra function calls.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>