ovpn: introduce the ovpn_socket object

ovpn: introduce the ovpn_socket object

This specific structure is used in the ovpn kernel module
to wrap and carry around a standard kernel socket.

ovpn takes ownership of passed sockets and therefore an ovpn
specific objects is attached to them for status tracking
purposes.

Initially only UDP support is introduced. TCP will come in a later
patch.

Cc: willemdebruijn.kernel@gmail.com
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Link: https://patch.msgid.link/20250415-b4-ovpn-v26-6-577f6097b964@openvpn.net
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>