ovpn: propagate socket mark to skb in UDP

ovpn: propagate socket mark to skb in UDP

OpenVPN allows users to configure a FW mark on sockets used to
communicate with other peers. The mark is set by means of the
`SO_MARK` Linux socket option.

However, in the ovpn UDP code path, the socket's `sk_mark` value is
currently ignored and it is not propagated to outgoing `skbs`.

This commit ensures proper inheritance of the field by setting
`skb->mark` to `sk->sk_mark` before handing the `skb` to the network
stack for transmission.

Fixes: 08857b5ec5d9 ("ovpn: implement basic TX path (UDP)")
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Link: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg31877.html
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>