ext4: optimize test_root()

The test_root() function could potentially loop forever due to
overflow issues.  So rewrite test_root() to avoid this issue; as a
bonus, it is 38% faster when benchmarked via a test loop:

int main(int argc, char **argv)
{
	int  i;

	for (i = 0; i < 1 << 24; i++) {
		if (test_root(i, 7))
			printf("%d\n", i);
	}
}
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/balloc.c

@@ -682,11 +682,15 @@ ext4_fsblk_t ext4_count_free_clusters(struct super_block *sb)
 
 static inline int test_root(ext4_group_t a, int b)
 {
-	int num = b;
-
-	while (a > num)
-		num *= b;
-	return num == a;
+	while (1) {
+		if (a < b)
+			return 0;
+		if (a == b)
+			return 1;
+		if ((a % b) != 0)
+			return 0;
+		a = a / b;
+	}
 }
 
 static int ext4_group_sparse(ext4_group_t group)