ftrace/selftests: Fix pid test with function graph not showing pids

The pid filtering test will set the pid filters and make sure that both
function and function_graph tracing honors the filters. But the
function_graph tracer test was failing because the PID was not being
filtered properly. That's because the funcgraph-proc option wasn't getting
set. Without that option the PID is not shown.

Instead we get:

	+ cat trace
	# tracer: function_graph
	#
	# CPU  DURATION                  FUNCTION CALLS
	# |     |   |                     |   |   |   |
	 3) ! 143.685 us  |  kernel_clone();
	 3) ! 127.055 us  |  kernel_clone();
	 1) ! 127.170 us  |  kernel_clone();
	 3) ! 126.840 us  |  kernel_clone();

When we should be getting:

	+ cat trace
	# tracer: function_graph
	#
	# CPU  TASK/PID         DURATION                  FUNCTION CALLS
	# |     |    |           |   |                     |   |   |   |
	 4)    bash-939    | # 1070.009 us |  kernel_clone();
	 4)    bash-939    | # 1116.903 us |  kernel_clone();
	 5)    bash-939    | ! 976.133 us  |  kernel_clone();
	 5)    bash-939    | ! 954.012 us  |  kernel_clone();

The test looks for the pids it is filtering and will fail if it can not
find them. Without fungraph-proc option set, it will not be displayed and
the test will fail.

Link: https://lore.kernel.org/all/Zl9JFnzKGuUM10X2@J2N7QTR9R3/
Link: https://lore.kernel.org/linux-trace-kernel/20240604152550.0c01d7cd@gandalf.local.home

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Fixes: 35b944a9 ("selftests/ftrace: Add function_graph tracer to func-filter-pid test")
Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Tested-by: Mark Rutland <mark.rutland@arm.com>

tools/testing/selftests/ftrace/test.d/ftrace/func-filter-pid.tc

@@ -8,6 +8,7 @@
 # Also test it on an instance directory
 
 do_function_fork=1
+do_funcgraph_proc=1
 
 if [ ! -f options/function-fork ]; then
     do_function_fork=0
@@ -28,6 +29,7 @@ fi
 
 if [ $do_funcgraph_proc -eq 1 ]; then
     orig_value2=`cat options/funcgraph-proc`
+    echo 1 > options/funcgraph-proc
 fi
 
 do_reset() {