perf/core: Don't release cred_guard_mutex if not taken

If we failed to acquire task's cred_guard_mutex we shouldn't proceed
to release it in the error path.

Fixes: a63fbed ("perf/tracing/cpuhotplug: Fix locking order")
Signed-off-by: Alexander Levin <alexander.levin@verizon.com>
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: mathieu.desnoyers@efficios.com
Cc: mhiramat@kernel.org
Cc: paulmck@linux.vnet.ibm.com
Cc: bigeasy@linutronix.de
Link: http://lkml.kernel.org/r/20170603033903.12056-1-alexander.levin@verizon.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/events/core.c

@@ -9878,7 +9878,7 @@ SYSCALL_DEFINE5(perf_event_open,
 	if (task) {
 		err = mutex_lock_interruptible(&task->signal->cred_guard_mutex);
 		if (err)
-			goto err_cred;
+			goto err_task;
 
 		/*
 		 * Reuse ptrace permission checks for now.