lib/rbtree_test.c: fix uninitialized variable warning

Fix this warning:

  lib/rbtree_test.c: In function `check':
  lib/rbtree_test.c:121: warning: `blacks' may be used uninitialized in this function
Signed-off-by: NCong Ding <dinggnu@gmail.com>
Cc: Michel Lespinasse <walken@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/rbtree_test.c

@@ -118,7 +118,7 @@ static void check(int nr_nodes)
 {
 	struct rb_node *rb;
 	int count = 0;
-	int blacks;
+	int blacks = 0;
 	u32 prev_key = 0;
 
 	for (rb = rb_first(&root); rb; rb = rb_next(rb)) {