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>
diff --git a/kernel/futex.c b/kernel/futex.c
index 60b47bb..62cbd64 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -227,6 +227,7 @@
 			return -EFAULT;
 		key->private.mm = mm;
 		key->private.address = address;
+		get_futex_key_refs(key);
 		return 0;
 	}