selftests: avoid using ifconfig

ifconfig is deprecated and not always present, use ip command instead.

Fixes: e0f3b3e5 ("selftests: Add test cases for vlan_filter modification during runtime")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Dong Chenchen <dongchenchen2@huawei.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20250730115313.3356036-1-edumazet@google.com


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