s390/runtime instrumentation: fix preemption imbalance

Commit 550435a ("s390/runtime instrumention: fix possible memory
corruption") introduced a preemption imbalance. Fix that!

That bug is not present in Linus' tree as that bug was fixed with the
rewrite in commit 8d9047f8b967 ("s390/runtime instrumentation: simplify
task exit handling"). However, that commit hasn't been backported.

Bad luck, Greg...

Found-by: Skimming over 'git diff v4.9.65..v4.9.66'
Signed-off-by: Mathias Krause <minipli@googlemail.com>

arch/s390/kernel/runtime_instr.c

@@ -49,10 +49,11 @@ void exit_thread_runtime_instr(void)
 
 	preempt_disable();
 	if (!task->thread.ri_cb)
-		return;
+		goto out;
 	disable_runtime_instr();
 	kfree(task->thread.ri_cb);
 	task->thread.ri_cb = NULL;
+out:
 	preempt_enable();
 }