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: Cong Ding <dinggnu@gmail.com>
Cc: Michel Lespinasse <walken@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>