selftests/ftrace: Use loopback address instead of localhost

Use raw loopback address instead of localhost, because
"localhost" can depend on nsswitch and in some case
we can not resolve the localhost.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>