perf_counter: Robustify counter-free logic

This fixes a nasty crash and highlights a bug that we were
freeing failed-fork() counters incorrectly.

(the fix for that will come separately)

[ Impact: fix crashes/lockups with inherited counters ]
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_counter.c

@@ -1004,6 +1004,10 @@ static void __perf_counter_task_sched_out(struct perf_counter_context *ctx)
 
 	if (!cpuctx->task_ctx)
 		return;
+
+	if (WARN_ON_ONCE(ctx != cpuctx->task_ctx))
+		return;
+
 	__perf_counter_sched_out(ctx, cpuctx);
 	cpuctx->task_ctx = NULL;
 }