selftests/net: packetdrill: Support single protocol test.

Currently, we cannot write IPv4 or IPv6 specific packetdrill tests
as ksft_runner.sh runs each .pkt file for both protocols.

Let's support single protocol test by checking --ip_version in the
.pkt file.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250819231527.1427361-1-kuniyu@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>