selftests/ftrace: Do not to use absolute debugfs path

selftests/ftrace: Do not to use absolute debugfs path

[ Upstream commit ba1b9c5048e43716921abe3a1db19cebebf4a5f5 ]

Use relative path to trigger file instead of absolute debugfs path,
because if the user uses tracefs instead of debugfs, it can be
mounted at /sys/kernel/tracing.
Anyway, since the ftracetest is designed to be run at the tracing
directory, user doesn't need to use absolute path.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>