Checksum-related utility functions

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

net_raw_checksum()
checksum calculation for a buffer

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Yan Vugenfirer <yan@daynix.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>