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

send_conn.go

@@ -10,6 +10,8 @@ type sendConn interface {
 	Close() error
 	LocalAddr() net.Addr
 	RemoteAddr() net.Addr
+
+	capabilities() connCapabilities
 }
 
 type sconn struct {