red: give a hint about burst value

Check for burst values that are too small.
Reported-by: Dave Taht <dave.taht@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>

tc/tc_red.c

@@ -56,8 +56,11 @@ int tc_red_eval_ewma(unsigned qmin, unsigned burst, unsigned avpkt)
 	double W = 0.5;
 	double a = (double)burst + 1 - (double)qmin/avpkt;
 
-	if (a < 1.0)
+	if (a < 1.0) {
+		fprintf(stderr, "tc_red_eval_ewma() burst %u is too small ?"
+				" Try burst %u\n", burst, 1 + qmin/avpkt);
 		return -1;
+	}
 	for (wlog=1; wlog<32; wlog++, W /= 2) {
 		if (a <= (1 - pow(1-W, burst))/W)
 			return wlog;