kselftests/ftrace: Detect tracefs mount point

Currently ftracetest assumes tracing directory is located under
$DEBUGFS/tracing.  But it's possible to mount tracefs directly without
debugfs.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>