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 <[e-mail]>
Acked-by: Steven Rostedt <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

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

···
 
 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