ovpn: add missing rtnl_link_ops->get_size callback

ovpn: add missing rtnl_link_ops->get_size callback

ovpn_fill_info emits IFLA_OVPN_MODE inside IFLA_INFO_DATA, but
ovpn_link_ops does not provide a get_size callback. Consequently,
rtnetlink's size estimate for ovpn link messages does not include the
nested mode attribute.

Available skb tailroom may hide this mismatch. When the remaining space
is insufficient, however, ovpn_fill_info returns -EMSGSIZE and message
construction fails.

Add the callback and account for IFLA_OVPN_MODE.

Fixes: c2d950c4672a ("ovpn: add basic interface creation/destruction/management routines")
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>