fix(unix): disable GSO after probing

After probing the given socket for GSO support, we need to disable this option
again to ensure we can selectively enable it via our cmsg codepaths.

quinn-udp/src/unix.rs

@@ -986,7 +986,15 @@ mod gso {
         // As defined in linux/udp.h
         // #define UDP_MAX_SEGMENTS        (1 << 6UL)
         match set_socket_option(socket, libc::SOL_UDP, libc::UDP_SEGMENT, GSO_SIZE) {
-            Ok(()) => 64,
+            Ok(()) => {
+                // Disable GSO again globally to ensure we can selectively enable it via cmsg.
+                // See:
+                // - https://github.com/quinn-rs/quinn/issues/2575
+                // - https://man7.org/linux/man-pages/man7/udp.7.html
+                let _ = set_socket_option(socket, libc::SOL_UDP, libc::UDP_SEGMENT, 0);
+
+                64
+            }
             Err(_e) => {
                 crate::log::debug!(
                     "failed to set `UDP_SEGMENT` socket option ({_e}); setting `max_gso_segments = 1`"