net: improve UDP/TCP checksum computation.

 * based on Eth, UDP, TCP struct present in eth.h instead of hardcoded
   indexes and sizes.
 * based on various macros present in eth.h.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Signed-off-by: Jason Wang <jasowang@redhat.com>
