futex: fixup get_futex_key() for private futexes

With the get_user_pages_fast() patches we made get_futex_key() obtain a
reference on the returned key, but failed to do so for private futexes.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>