net_checksum_add_cont()
checksum calculation for scattered data with odd chunk sizes

net_raw_checksum()
checksum calculation for a buffer
Signed-off-by: NDmitry Fleytman <dmitry@daynix.com>
Signed-off-by: NYan Vugenfirer <yan@daynix.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>