selftests: kprobe: Choose an always-defined function to probe

do_fork() is no longer defined on x86, so probe _do_fork() instead.

Fixes: 3033f14a ("clone: support passing tls argument via C ...")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>