selftests: forwarding: lib: Add helpers for checksum handling

In order to generate IGMPv3 and MLDv2 packets on the fly, we will need
helpers to calculate the packet checksum.

The approach presented in this patch revolves around payload templates
for mausezahn. These are mausezahn-like payload strings (01:23:45:...)
with possibly one 2-byte sequence replaced with the word PAYLOAD. The
main function is payload_template_calc_checksum(), which calculates
RFC 1071 checksum of the message. There are further helpers to then
convert the checksum to the payload format, and to expand it.

For IPv6, MLDv2 message checksum is computed using a pseudoheader that
differs from the header used in the payload itself. The fact that the
two messages are different means that the checksum needs to be
returned as a separate quantity, instead of being expanded in-place in
the payload itself. Furthermore, the pseudoheader includes a length of
the message. Much like the checksum, this needs to be expanded in
mausezahn format. And likewise for number of addresses for (S,G)
entries. Thus we have several places where a computed quantity needs
to be presented in the payload format. Add a helper u16_to_bytes(),
which will be used in all these cases.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

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