only run DPLPMTUD if the connection can send packets with the DF bit set (#3879)

sys_conn_df.go

@@ -4,9 +4,9 @@ package quic
 
 import "syscall"
 
-func setDF(rawConn syscall.RawConn) error {
+func setDF(syscall.RawConn) (bool, error) {
 	// no-op on unsupported platforms
-	return nil
+	return false, nil
 }
 
 func isMsgSizeErr(err error) bool {