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>

tools/testing/selftests/ftrace/test.d/kprobe/probepoint.tc

@@ -21,7 +21,7 @@ set_offs() { # prev target next
 
 # We have to decode symbol addresses to get correct offsets.
 # If the offset is not an instruction boundary, it cause -EILSEQ.
-set_offs `grep -A1 -B1 ${TARGET_FUNC} /proc/kallsyms | cut -f 1 -d " " | xargs`
+set_offs `grep -v __pfx_ /proc/kallsyms | grep -A1 -B1 ${TARGET_FUNC} | cut -f 1 -d " " | xargs`
 
 UINT_TEST=no
 # printf "%x" -1 returns (unsigned long)-1.