feat: add anti_gro setting to defeat TCP-GRO coalescing

[tracyhatemice]

Linux 6.18's PPPoE GRO offload (and any TCP-aware GRO path) coalesces a stream of mimic-produced fake-TCP packets into a single superframe, since mimic emits a constant wire ack_seq across a data burst. The receiver's XDP ingress then demangles the superframe as one packet, producing garbage that WireGuard drops -- collapsing ingress throughput.

New per-filter setting anti_gro (default off) makes egress add a per-packet random jitter to the wire ack_seq, which trips the flush check at net/ipv4/tcp_offload.c:337 (flush |= th->ack_seq ^ th2->ack_seq) and prevents coalescing in both the normal merge and fraglist paths. The padding entropy in conn_padding is adjusted to drop ack_seq when anti_gro is on, keeping receiver-side padding computation symmetric.

Both peers must enable the setting; default off preserves compatibility with prior mimic versions.

[hack3ric]

Thanks for the PR. But I have some questions:

• Will middlebox conntrack break because of it? I suspect either bpf_prandom_u32() is either too large as jitter, or ack_seq + some_random_number is not valid at all for a TCP connection. Mimic do have a test for conntrack behaviours; you can modify and test it to see if it works.
• Can we just disable TCP GRO on affected interfaces using ethtool or others? I think solving the root cause is better than a hack.

[airend]

• Will middlebox conntrack break because of it? I suspect either bpf_prandom_u32() is either too large as jitter, or ack_seq + some_random_number is not valid at all for a TCP connection. Mimic do have a test for conntrack behaviours; you can modify and test it to see if it works.

That's a big concern, that some firewalls (one use case) will be even more confused by these large nonlinear jumps. Also, would doing this increase the overhead a lot?

I can also report that this change breaks the BPF verifier on kernel 7.2-rc2 because it cannot prove that the padding length > 0 at the call site, so the program fails to load.