fix(s2n-quic-dc): include ipv6 header length in MAX_TOTAL_IPV6 payload length (#2727)

Author: boquan-fang

dc/s2n-quic-dc/src/msg/segment.rs

@@ -46,8 +46,8 @@ const MAX_TOTAL_IPV4: u16 = if cfg!(target_os = "linux") {
 
 /// The maximum payload allowed in sendmsg calls using IPv6+UDP
 const MAX_TOTAL_IPV6: u16 = if cfg!(target_os = "linux") {
-    // IPv6 doesn't include the IP header size in the calculation
-    u16::MAX - UDP_HEADER_LEN
+    // The IPV6_HEADER_LEN is required in this calculation to accommodate older kernels (such as kernel 5.10)
+    u16::MAX - IPV6_HEADER_LEN - UDP_HEADER_LEN
 } else {
     9001 - IPV6_HEADER_LEN - UDP_HEADER_LEN
 };