selftests: net: Fix printf format warnings on arm

Fix printf format warnings on arm (and other 32bit arch).

 - udpgso.c and udpgso_bench_tx use %lu for size_t but it
   should be unsigned long long on 32bit arch.

 - so_txtime.c uses %ld for int64_t, but it should be
   unsigned long long on 32bit arch.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Willem de Bruijn <willemb@google.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>