selftests/ftrace: Return unsupported if no error_log file

selftests/ftrace: Return unsupported if no error_log file

commit 619ee76f5c9f6a1d601d1a056a454d62bf676ae4 upstream.

Check whether error_log file exists in tracing/error_log testcase
and return UNSUPPORTED if no error_log file.

This can happen if we run the ftracetest on the older stable
kernel.

Fixes: 4eab1cc461a6 ("selftests/ftrace: Add tracing/error_log testcase")
Cc: stable@vger.kernel.org
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>