tap: Use tun's vnet-related code

tun and tap implements the same vnet-related features so reuse the code.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250207-tun-v6-7-fb49cf8b103e@daynix.com


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