ftracetest: Cope properly with stack tracer not being enabled

If the stack tracer (CONFIG_STACK_TRACER) is disabled, the
fgraph-filter-stack test blows chunks:

  [8] ftrace - function graph filters with stack tracer	[FAIL]
  + reset_tracer
  + echo nop
  ./ftracetest: 19: /home/michael/selftests/ftrace/test.d/ftrace/fgraph-filter-stack.tc:
  	cannot create /proc/sys/kernel/stack_tracer_enabled: Directory nonexistent

Fix it by checking if the proc file exists before echoing to it. With
the patch applied it fails correctly with:

  [8] ftrace - function graph filters with stack tracer	[UNSUPPORTED]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/ftrace/test.d/ftrace/fgraph-filter-stack.tc

@@ -16,7 +16,9 @@ fi
 
 do_reset() {
     reset_tracer
-    echo 0 > /proc/sys/kernel/stack_tracer_enabled
+    if [ -e /proc/sys/kernel/stack_tracer_enabled ]; then
+	    echo 0 > /proc/sys/kernel/stack_tracer_enabled
+    fi
     enable_tracing
     clear_trace
     echo > set_ftrace_filter