perf: Fix task_struct reference leak

sys_perf_event_open() had an imbalance in the number of task refs it
took causing memory leakage

Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: stable@kernel.org # .37+
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_event.c

@@ -6531,6 +6531,11 @@ SYSCALL_DEFINE5(perf_event_open,
 		goto err_alloc;
 	}
 
+	if (task) {
+		put_task_struct(task);
+		task = NULL;
+	}
+
 	/*
 	 * Look up the group leader (we will attach this event to it):
 	 */