netfilter: nf_tables: fix 'exist' matching on bigendian arches

netfilter: nf_tables: fix 'exist' matching on bigendian arches

Maze reports "tcp option fastopen exists" fails to match on
OpenWrt 22.03.5, r20134-5f15225c1e (5.10.176) router.

"tcp option fastopen exists" translates to:
inet
  [ exthdr load tcpopt 1b @ 34 + 0 present => reg 1 ]
  [ cmp eq reg 1 0x00000001 ]

.. but existing nft userspace generates a 1-byte compare.

On LSB (x86), "*reg32 = 1" is identical to nft_reg_store8(reg32, 1), but
not on MSB, which will place the 1 last. IOW, on bigendian aches the cmp8
is awalys false.

Make sure we store this in a consistent fashion, so existing userspace
will also work on MSB (bigendian).

Regardless of this patch we can also change nft userspace to generate
'reg32 == 0' and 'reg32 != 0' instead of u8 == 0 // u8 == 1 when
adding 'option x missing/exists' expressions as well.

Fixes: 3c1fece8819e ("netfilter: nft_exthdr: Allow checking TCP option presence, too")
Fixes: b9f9a485fb0e ("netfilter: nft_exthdr: add boolean DCCP option matching")
Fixes: 055c4b34b94f ("netfilter: nft_fib: Support existence check")
Reported-by: Maciej Żenczykowski <zenczykowski@gmail.com>
Closes: https://lore.kernel.org/netfilter-devel/CAHo-OozyEqHUjL2-ntATzeZOiuftLWZ_HU6TOM_js4qLfDEAJg@mail.gmail.com/
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>