futex: use fast_gup()

Change the get_user_pages() call with fast_gup() which doesn't require holding
the mmap_sem thereby removing the mmap_sem from all fast paths.
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>

kernel/futex.c

@@ -232,9 +232,7 @@ static int get_futex_key(u32 __user *uaddr, struct rw_semaphore *fshared,
 	}
 
 again:
-	down_read(&mm->mmap_sem);
-	err = get_user_pages(current, mm, address, 1, 0, 0, &page, NULL);
-	up_read(&mm->mmap_sem);
+	err = get_user_pages_fast(address, 1, 0, &page);
 	if (err < 0)
 		return err;