random: absorb fast pool into input pool after fast load

During crng_init == 0, we never credit entropy in add_interrupt_
randomness(), but instead dump it directly into the primary_crng. That's
fine, except for the fact that we then wind up throwing away that
entropy later when we switch to extracting from the input pool and
xoring into (and later in this series overwriting) the primary_crng key.
The two other early init sites -- add_hwgenerator_randomness()'s use
crng_fast_load() and add_device_ randomness()'s use of crng_slow_load()
-- always additionally give their inputs to the input pool. But not
add_interrupt_randomness().

This commit fixes that shortcoming by calling mix_pool_bytes() after
crng_fast_load() in add_interrupt_randomness(). That's partially
verboten on PREEMPT_RT, where it implies taking spinlock_t from an IRQ
handler. But this also only happens during early boot and then never
again after that. Plus it's a trylock so it has the same considerations
as calling crng_fast_load(), which we're already using.

Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Suggested-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -850,6 +850,10 @@ void add_interrupt_randomness(int irq)
 		    crng_fast_load((u8 *)fast_pool->pool, sizeof(fast_pool->pool)) > 0) {
 			fast_pool->count = 0;
 			fast_pool->last = now;
+			if (spin_trylock(&input_pool.lock)) {
+				_mix_pool_bytes(&fast_pool->pool, sizeof(fast_pool->pool));
+				spin_unlock(&input_pool.lock);
+			}
 		}
 		return;
 	}