pppol2tp: calls unregister_pernet_gen_device() at unload time

Failure to call unregister_pernet_gen_device() can exhaust memory
if module is loaded/unloaded many times.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>