selftests: net: simple selftest for ipvtap

selftests: net: simple selftest for ipvtap

This is a simple ipvtap test to test handling
IP-address add/remove on ipvlan interface.

It creates a veth-interface and then creates several
network-namespace with ipvlan0 interface in it linked to veth.

Then it starts to add/remove addresses on ipvlan0 interfaces
in several threads.

At finish, it checks that there is no duplicated addresses.

Signed-off-by: Dmitry Skorodumov <skorodumov.dmitry@huawei.com>
Link: https://patch.msgid.link/20260112142417.4039566-3-skorodumov.dmitry@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>