src: raw payload match and mangle on inner header / payload data

src: raw payload match and mangle on inner header / payload data

This patch adds support to match on inner header / payload data:

 # nft add rule x y @ih,32,32 0x14000000 counter

you can also mangle payload data:

 # nft add rule x y @ih,32,32 set 0x14000000 counter

This update triggers a checksum update at the layer 4 header via
csum_flags, mangling odd bytes is also aligned to 16-bits.

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