l2tp: hold tunnel while handling genl tunnel updates

We need to make sure the tunnel is not going to be destroyed by
l2tp_tunnel_destruct() concurrently.

Fixes: 309795f4

 ("l2tp: Add netlink control API for L2TP")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>