ovpn: ensure socket is owned by ovpn before deref sk_user_data

ovpn: ensure socket is owned by ovpn before deref sk_user_data

Some subsystems, like BPF SOCKMAP, set sk_user_data without
actually setting the encap_type.

For this reason, we must make sure that the type is the
one ovpn expects before dereferencing sk_user_data.

Failing to do so may lead to out-of-bounds reads.

Fixes: f6226ae7a0cd ("ovpn: introduce the ovpn_socket object")
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>