selftests/net: udpgso_bench_tx: fix dst ip argument

selftests/net: udpgso_bench_tx: fix dst ip argument

udpgso_bench_tx call setup_sockaddr() for dest address before
parsing all arguments, if we specify "-p ${dst_port}" after "-D ${dst_ip}",
then ${dst_port} will be ignored, and using default cfg_port 8000.

This will cause test case "multiple GRO socks" failed in udpgro.sh.

Setup sockaddr after parsing all arguments.

Fixes: 4d4a47023538 ("selftests: udp gso benchmark")
Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/ff620d9f-5b52-06ab-5286-44b945453002@163.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>