payload: only merge if adjacent and combined size fits into a register

payload: only merge if adjacent and combined size fits into a register

add rule ip6 filter input ip6 saddr ::1/128 ip6 daddr ::1/128 fails,
we ask to compare a 32byte immediate which is not supported:

  [ payload load 32b @ network header + 8 => reg 1 ]
  [ cmp eq reg 1 0x00000000 0x00000000 0x00000000 0x01000000 0x00000000 0x00000000 0x00000000 0x02000000 ]

We would need to use two cmps in this case, i.e.:

  [ payload load 32b @ network header + 8 => reg 1 ]
  [ cmp eq reg 1 0x00000000 0x00000000 0x00000000 0x01000000 ]
  [ cmp eq reg 2 0x00000000 0x00000000 0x00000000 0x02000000 ]

Seems however that this requires a bit more changes to how nft
handles register allocations, we'd also need to undo the constant merge.

Lets disable merging for now so that we generate

  [ payload load 16b @ network header + 8 => reg 1 ]
  [ cmp eq reg 1 0x00000000 0x00000000 0x00000000 0x01000000 ]
  [ payload load 16b @ network header + 24 => reg 1 ]
  [ cmp eq reg 1 0x00000000 0x00000000 0x00000000 0x02000000 ]

... if merge would bring us over the 128 bit register size.

Closes: http://bugzilla.netfilter.org/show_bug.cgi?id=1032
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>