The kernel leaks memory when a `fib` rule is present in ipv6 nftables firewall rules and a suppress_prefix rule is present in the IPv6 routing rules (used by certain tools such as wg-quick). In such scenarios, every incoming packet will leak an allocation in ip6_dst_cache slab cache.

After some hours of `bpftrace`-ing and source code reading, I tracked down the issue to this commit: https://github.com/torvalds/linux/commit/ca7a03c4175366a92cee0ccc4fec0038c3266e26

The problem with that change is that the generic args->flags always have FIB_LOOKUP_NOREF set[1][2] but the ip6-specific flag RT6_LOOKUP_F_DST_NOREF might not be, leading to fib6_rule_suppress not decreasing the refcount when needed.

How to reproduce:
 - Add the following nftables rule to a prerouting chain: `meta nfproto ipv6 fib saddr . mark . iif oif missing drop`. Can be done with:
    sudo nft create table inet test
    sudo nft create chain inet test test_chain '{ type filter hook prerouting priority filter + 10; policy accept; }'
    sudo nft add rule inet test test_chain meta nfproto ipv6 fib saddr . mark . iif oif missing drop
 - Run `sudo ip -6 rule add table main suppress_prefixlength 0`
 - Watch `sudo slabtop -o | grep ip6_dst_cache` memory usage increase with every incoming ipv6 packet

Suggested fix: Expose the protocol-specific flags to the protocol specific `suppress` function, and check the protocol-specific `flags` argument for RT6_LOOKUP_F_DST_NOREF instead of the generic FIB_LOOKUP_NOREF when decreasing the refcount, like this: https://gist.github.com/msizanoen1/36a2853467a9bd34fadc5bb3783fde0f

[1]: https://github.com/torvalds/linux/blob/ca7a03c4175366a92cee0ccc4fec0038c3266e26/net/ipv6/fib6_rules.c#L71
[2]: https://github.com/torvalds/linux/blob/ca7a03c4175366a92cee0ccc4fec0038c3266e26/net/ipv6/fib6_rules.c#L99