datapath: Harmonize [get|set]_skb_csum_pointers().

datapath: Harmonize [get|set]_skb_csum_pointers().

The functions to get and set the checksum pointers consistently across
different kernel versions had different interpretations of what the
csum_offset pointer was relative to, which is confusing, to say the least.
This makes the meaning be the same as skb->csum_offset in modern kernels
and updates the caller.  For a given function the results were consistent
across kernel versions and the callers knew what the meaning should be, so
this doesn't actually fix any bugs.

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