selftests/ftrace: check for do_sys_openat2 in user-memory test

More recent libc implementations are now using openat/openat2 system
calls so also add do_sys_openat2 to the tracing so that the test
passes on these systems because do_sys_open may not be called.

Thanks to Masami Hiramatsu for the help on getting this fix to work
correctly.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>