tracing/selftests: Fix kprobe event name test for .isra. functions

The kprobe_eventname.tc test checks if a function with .isra. can have a
kprobe attached to it. It loops through the kallsyms file for all the
functions that have the .isra. name, and checks if it exists in the
available_filter_functions file, and if it does, it uses it to attach a
kprobe to it.

The issue is that kprobes can not attach to functions that are listed more
than once in available_filter_functions. With the latest kernel, the
function that is found is: rapl_event_update.isra.0

  # grep rapl_event_update.isra.0 /sys/kernel/tracing/available_filter_functions
  rapl_event_update.isra.0
  rapl_event_update.isra.0

It is listed twice. This causes the attached kprobe to it to fail which in
turn fails the test. Instead of just picking the function function that is
found in available_filter_functions, pick the first one that is listed
only once in available_filter_functions.

Cc: stable@vger.kernel.org
Fixes: 604e3548

 ("selftests/ftrace: Select an existing function in kprobe_eventname test")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>