kill checksum offloading in sk.

kill checksum offloading in sk.
it was RX only, nothing on the TX side
rather complex, might eat up (or more) offloading benefits in many cases
the hardware miscomputes the cksums sometimes, so we don't trust it
claiming a cksum is bad and re-do the cksum verification in sw then...
(but we trust it to not mark bad ones good? hmmmmmmmmm.)
diff was on tech for 3 months, nobody disagreed, everybody who spoke up
agrees or doesn't care.
pretty exactly a revert of the cksum offloading addition years ago, tested
by several, the only report i quickly find now is from David Higgs <higgsd
at gmail dot com>, thanks!