rbtree test: fix sparse warning about 64-bit constant

Just a small fix to make sparse happy.

Signed-off-by: Michel Lespinasse <walken@google.com>
Reported-by: Fengguang Wu <wfg@linux.intel.com>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>