random: cap jitter samples per bit to factor of HZ

Currently the jitter mechanism will require two timer ticks per
iteration, and it requires N iterations per bit. This N is determined
with a small measurement, and if it's too big, it won't waste time with
jitter entropy because it'd take too long or not have sufficient entropy
anyway.

With the current max N of 32, there are large timeouts on systems with a
small CONFIG_HZ. Rather than set that maximum to 32, instead choose a
factor of CONFIG_HZ. In this case, 1/30 seems to yield sane values for
different configurations of CONFIG_HZ.

Reported-by: Vladimir Murzin <vladimir.murzin@arm.com>
Fixes: 78c768e ("random: vary jitter iterations based on cycle counter speed")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/random.c

@@ -1174,7 +1174,7 @@ static void __cold entropy_timer(struct timer_list *timer)
  */
 static void __cold try_to_generate_entropy(void)
 {
-	enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = 32 };
+	enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 30 };
 	struct entropy_timer_state stack;
 	unsigned int i, num_different = 0;
 	unsigned long last = random_get_entropy();