x86, bts: fix unlock problem in ds.c

Fix a problem where ds_request() returned an error without releasing the
ds lock.
Reported-by: NStephane Eranian <eranian@gmail.com>
Signed-off-by: NMarkus Metzger <markus.t.metzger@gmail.com>
Cc: <stable@kernel.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/ds.c

@@ -384,8 +384,9 @@ static int ds_request(struct task_struct *task, void *base, size_t size,
 
 	spin_lock(&ds_lock);
 
+	error = -EPERM;
 	if (!check_tracer(task))
-		return -EPERM;
+		goto out_unlock;
 
 	error = -ENOMEM;
 	context = ds_alloc_context(task);