BUG/MINOR: stconn: Increase SC bytes_out value in se_done_ff()

When data are sent via the zero-copy data forwarding, we must not forget to
increase the stconn bytes_out value.

This patch must be backport to 3.3.

include/haproxy/stconn.h

@@ -525,7 +525,7 @@ static inline size_t se_done_ff(struct sedesc *se)
 			}
 		}
 	}
-
+	se->sc->bytes_out += ret;
 	return ret;
 }