nftables: xt: fix misprint in nft_xt_compatible_revision

nftables: xt: fix misprint in nft_xt_compatible_revision

The rev variable is used here instead of opt obviously by mistake.
Please see iptables:nft_compatible_revision() for an example how it
should be.

This breaks revision compatibility checks completely when reading
compat-target rules from nft utility. That's why nftables can't work on
"old" kernels which don't support new revisons. That's a problem for
containers.

E.g.: 0 and 1 is supported but not 2:
https://git.sw.ru/projects/VZS/repos/vzkernel/browse/net/netfilter/xt_nat.c#111

Reproduce of the problem on Virtuozzo 7 kernel
3.10.0-1160.11.1.vz7.172.18 in centos 8 container:

  iptables-nft -t nat -N TEST
  iptables-nft -t nat -A TEST -j DNAT --to-destination 172.19.0.2
  nft list ruleset > nft.ruleset
  nft -f - < nft.ruleset
  #/dev/stdin:19:67-81: Error: Range has zero or negative size
  #		meta l4proto tcp tcp dport 81 counter packets 0 bytes 0 dnat to 3.0.0.0-0.0.0.0
  #		                                                                ^^^^^^^^^^^^^^^

  nft -v
  #nftables v0.9.3 (Topsy)
  iptables-nft -v
  #iptables v1.8.7 (nf_tables)

Kernel returns ip range in rev 0 format:

  crash> p *((struct nf_nat_ipv4_multi_range_compat *) 0xffff8ca2fabb3068)
  $5 = {
    rangesize = 1,
    range = {{
        flags = 3,
        min_ip = 33559468,
        max_ip = 33559468,

But nft reads this as rev 2 format (nf_nat_range2) which does not have
rangesize, and thus flugs 3 is treated as ip 3.0.0.0, which is wrong and
can't be restored later.

(Should probably be the same on Centos 7 kernel 3.10.0-1160.11.1)

Fixes: fbc0768cb696 ("nftables: xt: don't use hard-coded AF_INET")
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>