ovpn: fix use after free in unlock_ovpn()

ovpn: fix use after free in unlock_ovpn()

unlock_ovpn() iterates over the release_list using llist_for_each_entry()
and drops the peer reference inside the loop body via ovpn_peer_put().

If this drops the last reference, the peer is eventually freed. However,
llist_for_each_entry() reads peer->release_entry.next in the loop advance
expression, which runs after the body. By that time the peer may have
already been freed, resulting in a use after free when advancing to the
next list entry.

Fix this by using llist_for_each_entry_safe(), which caches the next
pointer before executing the loop body.

Fixes: 80747caef33d ("ovpn: introduce the ovpn_peer object")
Signed-off-by: Marco Baffo <marco@mandelbit.com>
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>