random: throttle hwrng writes if no entropy is credited

If a hwrng source does not provide an entropy estimate, it currently
does not contribute at all to the CRNG. In order to help fix this, in
case add_hwgenerator_randomness() is called with the entropy parameter
set to zero, go to sleep until one reseed interval has passed.

While the hwrng thread currently only runs under conditions where this
is non-zero, this change is not harmful and prepares for future updates
to the hwrng core.

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -865,9 +865,9 @@ void add_hwgenerator_randomness(const void *buf, size_t len, size_t entropy)
 
 	/*
 	 * Throttle writing to once every reseed interval, unless we're not yet
-	 * initialized.
+	 * initialized or no entropy is credited.
 	 */
-	if (!kthread_should_stop() && crng_ready())
+	if (!kthread_should_stop() && (crng_ready() || !entropy))
 		schedule_timeout_interruptible(crng_reseed_interval());
 }
 EXPORT_SYMBOL_GPL(add_hwgenerator_randomness);