selftests/ftrace: Reset ftrace filter on older kernel

Since older kernel didn't support separated instance of
set_ftrace_filter, if the test case set the filter in
an instance, it will propagate to top-level instance.
This means that the filter setting remains even if we
remove the instance, and will cause other tests failure.
To avoid this issue, reset the ftrace filter if we
detect the propagation.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>