selftests/ftrace: Fix eventfs ownership testcase to find mount point

Fix eventfs ownership testcase to find mount point if stat -c "%m" failed.
This can happen on the system based on busybox. In this case, this will
try to use the current working directory, which should be a tracefs top
directory (and eventfs is mounted as a part of tracefs.)
If it does not work, the test is skipped as UNRESOLVED because of
the environmental problem.

Fixes: ee9793be ("tracing/selftests: Add ownership modification tests for eventfs")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>