netfilter: add CHECKSUM target

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



Typical expected use (helps old dhclient binary running in a VM):
iptables -A POSTROUTING -t mangle -p udp --dport bootpc \
	-j CHECKSUM --checksum-fill

Includes fixes by Jan Engelhardt <jengelh@medozas.de>

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>