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

sys_conn_no_oob.go

@@ -4,8 +4,8 @@ package quic
 
 import "net"
 
-func newConn(c net.PacketConn) (rawConn, error) {
-	return &basicConn{PacketConn: c}, nil
+func newConn(c net.PacketConn, supportsDF bool) (rawConn, error) {
+	return &basicConn{PacketConn: c, supportsDF: supportsDF}, nil
 }
 
 func inspectReadBuffer(any) (int, error)  { return 0, nil }