ovpn: fix possible use-after-free in ovpn_net_xmit

In the Linux kernel, the following vulnerability has been resolved:

ovpn: fix possible use-after-free in ovpn_net_xmit

When building the skb_list in ovpn_net_xmit, skb_share_check will free

the original skb if it is shared. The current implementation continues

to use the stale skb pointer for subsequent operations:

- peer lookup,

- skb_dst_drop (even though all segments produced by skb_gso_segment

will have a dst attached),

- ovpn_peer_stats_increment_tx.

Fix this by moving the peer lookup and skb_dst_drop before segmentation

so that the original skb is still valid when used. Return early if all

segments fail skb_share_check and the list ends up empty.

Also switch ovpn_peer_stats_increment_tx to use skb_list.next; the next

patch fixes the stats logic.