random: schedule jitter credit for next jiffy, not in two jiffies

Counterintuitively, mod_timer(..., jiffies + 1) will cause the timer to
fire not in the next jiffy, but in two jiffies. The way to cause
the timer to fire in the next jiffy is with mod_timer(..., jiffies).
Doing so then lets us bump the upper bound back up again.

Fixes: 50ee7529ec ("random: try to actively add entropy rather than passively wait for it")
Fixes: 829d680e82 ("random: cap jitter samples per bit to factor of HZ")
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -1205,7 +1205,7 @@ 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 = HZ / 30 };
+	enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 15 };
 	struct entropy_timer_state stack;
 	unsigned int i, num_different = 0;
 	unsigned long last = random_get_entropy();
@@ -1224,7 +1224,7 @@ static void __cold try_to_generate_entropy(void)
 	timer_setup_on_stack(&stack.timer, entropy_timer, 0);
 	while (!crng_ready() && !signal_pending(current)) {
 		if (!timer_pending(&stack.timer))
-			mod_timer(&stack.timer, jiffies + 1);
+			mod_timer(&stack.timer, jiffies);
 		mix_pool_bytes(&stack.entropy, sizeof(stack.entropy));
 		schedule();
 		stack.entropy = random_get_entropy();