Convert partially summed packets to be fully checksummed.

In case csum offloaded packet, vmxnet3 implementation always passes an
RxCompDesc with the "Checksum calculated and found correct" notification
to the OS. This emulates the observed ESXi behavior.

Therefore, if packet has the NEEDS_CSUM bit set, we must calculate and
place a fully computed checksum into the tcp/udp header. Otherwise, the
OS driver will receive a checksum-correct indication but with the actual
tcp/udp checksum field having just the pseudo header csum value.

If host OS performs forwarding, it will forward an incorrectly
checksummed packet.
Signed-off-by: NDana Rubin <dana.rubin@ravellosystems.com>
Signed-off-by: NShmulik Ladkani <shmulik.ladkani@ravellosystems.com>
Message-id: 1436864116-19154-3-git-send-email-shmulik.ladkani@ravellosystems.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>