selftests: drv-net: get detailed interface info

We already record output of ip link for NETIF in env for easy access.
Record the detailed version. TSO test will want to know the max tso size.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20250218225426.77726-3-kuba@kernel.org


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