xt_CHECKSUM: initial import

xt_CHECKSUM: initial import

This adds a "CHECKSUM" target, which can be used in the iptables mangle
table.

You can use this target to compute and fill in the checksum in a packet
that lacks a checksum. This is particularly useful, if you need to work
around old applications such as dhcp clients, that do not work well with
checksum offloads, but don't want to disable checksum offload in your
device.

The problem happens in the field with virtualized applications. For
reference, see Red Hat bz 605555, as well as
http://www.spinics.net/lists/kvm/msg37660.html

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>