batman-adv: fix batadv_nc_random_weight_tq

and change to pseudorandom numbers, as this is a traffic dithering
operation that doesn't need crypto-grade.

The previous code operated in 4 steps:

1. Generate a random byte 0 <= rand_tq <= 255
2. Multiply it by BATADV_TQ_MAX_VALUE - tq
3. Divide by 255 (= BATADV_TQ_MAX_VALUE)
4. Return BATADV_TQ_MAX_VALUE - rand_tq

This would apperar to scale (BATADV_TQ_MAX_VALUE - tq) by a random
value between 0/255 and 255/255.

But!  The intermediate value between steps 3 and 4 is stored in a u8
variable.  So it's truncated, and most of the time, is less than 255, after
which the division produces 0.  Specifically, if tq is odd, the product is
always even, and can never be 255.  If tq is even, there's exactly one
random byte value that will produce a product byte of 255.

Thus, the return value is 255 (511/512 of the time) or 254 (1/512
of the time).

If we assume that the truncation is a bug, and the code is meant to scale
the input, a simpler way of looking at it is that it's returning a random
value between tq and BATADV_TQ_MAX_VALUE, inclusive.

Well, we have an optimized function for doing just that.

Fixes: 3c12de9a5c ("batman-adv: network coding - code and transmit packets if possible")
Signed-off-by: George Spelvin <lkml@sdf.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/network-coding.c

@@ -1009,15 +1009,8 @@ static struct batadv_nc_path *batadv_nc_get_path(struct batadv_priv *bat_priv,
  */
 static u8 batadv_nc_random_weight_tq(u8 tq)
 {
-	u8 rand_val, rand_tq;
-
-	get_random_bytes(&rand_val, sizeof(rand_val));
-
 	/* randomize the estimated packet loss (max TQ - estimated TQ) */
-	rand_tq = rand_val * (BATADV_TQ_MAX_VALUE - tq);
-
-	/* normalize the randomized packet loss */
-	rand_tq /= BATADV_TQ_MAX_VALUE;
+	u8 rand_tq = prandom_u32_max(BATADV_TQ_MAX_VALUE + 1 - tq);
 
 	/* convert to (randomized) estimated tq again */
 	return BATADV_TQ_MAX_VALUE - rand_tq;