Fixed the bug of missing expression numgen from GetRules

[sanya2022]

When we create a rule with expression numgen, it is created correctly. Here's an example:

table inet james-filter {
	map m2798997459 {
		type 0 : ipv4_addr
		flags constant
		elements = { 0 : 1.1.1.1, 1 : 2.2.2.2, 2 : 3.3.3.3 }
	}
	chain input {
		type nat hook prerouting priority filter; policy accept;
		ip saddr 1.1.1.1 th dport 2222 log prefix "nflog" group 0 counter packets 0 bytes 0 dnat ip to numgen inc mod 3 map @m2798997459 comment "temp rule"
	}
}

However, when we get the same rule from netlink via this library, the expression numgen is missing:

2025/01/31 20:52:41 *expr.Meta &{15 false 1}
2025/01/31 20:52:41 *expr.Cmp &{0 1 [2]}
2025/01/31 20:52:41 *expr.Payload &{0 1 0 1 12 4 0 0 0}
2025/01/31 20:52:41 *expr.Cmp &{0 1 [1 1 1 1]}
2025/01/31 20:52:41 *expr.Payload &{0 1 0 2 2 2 0 0 0}
2025/01/31 20:52:41 *expr.Cmp &{0 1 [8 174]}
2025/01/31 20:52:41 *expr.Log &{0 0 6 0 0 0 [110 102 108 111 103]}
2025/01/31 20:52:41 *expr.Counter &{0 0}
2025/01/31 20:52:41 *expr.Lookup &{1 1 true 0 m2798997459 false}
2025/01/31 20:52:41 *expr.NAT &{1 2 1 1 0 0 false false false false false}

Example with fix:

2025/01/31 20:52:14 *expr.Meta &{15 false 1}
2025/01/31 20:52:14 *expr.Cmp &{0 1 [2]}
2025/01/31 20:52:14 *expr.Payload &{0 1 0 1 12 4 0 0 0}
2025/01/31 20:52:14 *expr.Cmp &{0 1 [1 1 1 1]}
2025/01/31 20:52:14 *expr.Payload &{0 1 0 2 2 2 0 0 0}
2025/01/31 20:52:14 *expr.Cmp &{0 1 [8 174]}
2025/01/31 20:52:14 *expr.Log &{0 0 6 0 0 0 [110 102 108 111 103]}
2025/01/31 20:52:14 *expr.Counter &{0 0}
2025/01/31 20:52:14 *expr.Numgen &{1 3 0 0}
2025/01/31 20:52:14 *expr.Lookup &{1 1 true 0 m2798997459 false}
2025/01/31 20:52:14 *expr.NAT &{1 2 1 1 0 0 false false false false false}

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[stapelberg]

Seems simple enough, but I can’t accept this PR before you sign the CLA. Please let me know once you signed it and the PR is ready for review. Thanks!

[sanya2022]

@stapelberg Done it!