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

kernel/futex.c

@@ -227,6 +227,7 @@ static int get_futex_key(u32 __user *uaddr, int fshared, union futex_key *key)
 			return -EFAULT;
 		key->private.mm = mm;
 		key->private.address = address;
+		get_futex_key_refs(key);
 		return 0;
 	}