function trace: fix a bug of single thread function trace

Impact: fix "no output from tracer" bug caused by ftrace_update_pid_func()

When disabling single thread function trace using
"echo -1 > set_ftrace_pid", the normal function trace
has to restore to original function, otherwise the normal
function trace will not work well.

Without this commit, something like below:

	$ ps |grep 850
	  850 root      2556 S    -/bin/sh
	$ echo 850 > /debug/tracing/set_ftrace_pid
	$ echo function > /debug/tracing/current_tracer
	$ echo 1 > /debug/tracing/tracing_enabled
	$ sleep 1
	$ echo 0 > /debug/tracing/tracing_enabled
	$ cat /debug/tracing/trace_pipe |wc -l
	59704
	$ echo -1 > /debug/tracing/set_ftrace_pid
	$ echo 1 > /debug/tracing/tracing_enabled
	$ sleep 1
	$ echo 0 > /debug/tracing/tracing_enabled
	$ more /debug/tracing/trace_pipe
		<====== nothing output now!
			it should output trace record.
Signed-off-by: Liming Wang <liming.wang@windriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/ftrace.c

@@ -243,10 +243,8 @@ static void ftrace_update_pid_func(void)
 		set_ftrace_pid_function(func);
 		func = ftrace_pid_func;
 	} else {
-		if (func != ftrace_pid_func)
-			goto out;
-
-		set_ftrace_pid_function(func);
+		if (func == ftrace_pid_func)
+			func = ftrace_pid_function;
 	}
 
 #ifdef CONFIG_HAVE_FUNCTION_TRACE_MCOUNT_TEST