netfilter: nft_payload: layer 4 checksum adjustment for pseudoheader fields

This patch adds a new flag that signals the kernel to update layer 4
checksum if the packet field belongs to the layer 4 pseudoheader. This
implicitly provides stateless NAT 1:1 that is useful under very specific
usecases.

Since rules mangling layer 3 fields that are part of the pseudoheader
may potentially convey any layer 4 packet, we have to deal with the
layer 4 checksum adjustment using protocol specific code.

This patch adds support for TCP, UDP and ICMPv6, since they include the
pseudoheader in the layer 4 checksum calculation. ICMP doesn't, so we
can skip it.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>