selftests/ftrace: Fix probepoint testcase to ignore __pfx_* symbols

Fix kprobe probepoint testcase to ignore __pfx_* prefix symbols. Those are
introduced by commit b341b20d ("x86: Add prefix symbols for function
padding") for identifying PADDING_BYTES of NOPs. Since kprobe events can
not probe these prefix symbols, this testcase has to skip those symbols.

Link: https://lore.kernel.org/all/167309835609.640500.9664678940260305746.stgit@devnote3/



Fixes: b341b20d ("x86: Add prefix symbols for function padding")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>