datapath: Try to avoid custom checksum update function.

datapath: Try to avoid custom checksum update function.

Our update_csum() function was exactly the same as
inet_proto_csum_replace4() with the one exception that it uses our
checksum status fields on older kernels that need it.  Unfortunately,
we can't completely move the code to the compat directory because it
relies on fields in OVS CB but we can at least exile it to checksum.h.

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