selftests/ftrace: Use long for synthetic event probe test

On 32bit the trigger-synthetic-eprobe.tc selftest fails with the error:

hist:syscalls:sys_exit_openat: error: Param type doesn't match synthetic event field type
  Command: hist:keys=common_pid:filename=$__arg__1,ret=ret:onmatch(syscalls.sys_enter_openat).trace(synth_open,$filename,$ret)
                                                                                                               ^
This is because the synth_open synthetic event is created with:

  echo "$SYNTH u64 filename; s64 ret;" > synthetic_events

Which works fine on 64 bit, as filename is a pointer and the return is
also a long. But for 32 bit architectures, it doesn't work.

Use "unsigned long" and "long" instead so that it works for both 64 bit
and 32 bit architectures.

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>