random: simplify arithmetic function flow in account()

Now that have_bytes is never modified, we can simplify this function.
First, we move the check for negative entropy_count to be first. That
ensures that subsequent reads of this will be non-negative. Then,
have_bytes and ibytes can be folded into their one use site in the
min_t() function.
Suggested-by: Dominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -1293,7 +1293,7 @@ EXPORT_SYMBOL_GPL(add_disk_randomness);
  */
 static size_t account(size_t nbytes, int min)
 {
-	int entropy_count, orig, have_bytes;
+	int entropy_count, orig;
 	size_t ibytes, nfrac;
 
 	BUG_ON(input_pool.entropy_count > POOL_FRACBITS);
@@ -1301,20 +1301,15 @@ static size_t account(size_t nbytes, int min)
 	/* Can we pull enough? */
 retry:
 	entropy_count = orig = READ_ONCE(input_pool.entropy_count);
-	ibytes = nbytes;
-	/* never pull more than available */
-	have_bytes = entropy_count >> (POOL_ENTROPY_SHIFT + 3);
-
-	if (have_bytes < 0)
-		have_bytes = 0;
-	ibytes = min_t(size_t, ibytes, have_bytes);
-	if (ibytes < min)
-		ibytes = 0;
-
 	if (WARN_ON(entropy_count < 0)) {
 		pr_warn("negative entropy count: count %d\n", entropy_count);
 		entropy_count = 0;
 	}
+
+	/* never pull more than available */
+	ibytes = min_t(size_t, nbytes, entropy_count >> (POOL_ENTROPY_SHIFT + 3));
+	if (ibytes < min)
+		ibytes = 0;
 	nfrac = ibytes << (POOL_ENTROPY_SHIFT + 3);
 	if ((size_t)entropy_count > nfrac)
 		entropy_count -= nfrac;