ovpn: don't drop skb's dst when xmitting packet

ovpn: don't drop skb's dst when xmitting packet

When routing a packet to a LAN behind a peer, ovpn needs to
inspect the route entry that brought the packet there in the
first place.

If this packet is truly routable, the route entry provides the
GW to be used when looking up the VPN peer to send the packet to.

However, the route entry is currently dropped before entering
the ovpn xmit function, because the IFF_XMIT_DST_RELEASE priv_flag
is enabled by default.

Clear the IFF_XMIT_DST_RELEASE flag during interface setup to allow
the route entry (skb's dst) to survive and thus be inspected
by the ovpn routing logic.

Fixes: a3aaef8cd173 ("ovpn: implement peer lookup logic")
Reported-by: Gert Doering <gert@greenie.muc.de>
Closes: https://github.com/OpenVPN/ovpn-net-next/issues/2
Tested-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Gert Doering <gert@greenie.muc.de> # as a primary user
Link: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg31583.html
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>