fix(s2n-quic-dc): include ipv6 header length in MAX_TOTAL_IPV6 payload length

[boquan-fang]

Release Summary:

Resolved issues:

Description of changes:

We should include a 40 bytes IPV6_HEADER_LEN in the calculation for the max payload size that can be sent for IPV6.

I was testing this code on AL2 X86_64 Kernel 5.10 machine, and the max_total_test is consistently failing. I test the limit by changing the constant definition of MAX_TOTAL_IPV6, and found out that the maximum amount that can be sent with IPV6 is u16::MAX - IPV6_HEADER_LEN - UDP_HEADER_LEN, the current calculation doesn't include IPV6_HEADER_LEN.

Call-outs:

This problem only happens on AL2 X86_64 Kernel 5.10 machine, and we are not aware of any other platforms that will make this fail.

The next step for this PR is to investigate issue #2728.

Testing:

I ran the code on AL2 X86_64 Kernel 5.10 machine, and it succeeded:

(25-07-25 20:53:59) <0> [~/workspace/s2n-quic/dc/s2n-quic-dc/src]  
machine % uname -r
5.10.238-215.956.amzn2int.x86_64

(25-07-25 20:54:01) <0> [~/workspace/s2n-quic/dc/s2n-quic-dc/src]  
machine % cargo test max_total_test
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.11s
     Running unittests src/lib.rs (/workplace/bqfang/s2n-quic/target/debug/deps/s2n_quic_dc-456121e1fd155bb6)

running 1 test
test msg::segment::max_total_test ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 328 filtered out; finished in 0.00s

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.