l2tp: fix refcount leak in l2tp_ip sockets

The l2tp_ip socket close handler does not update the module refcount
correctly which prevents module unload after the first bind() call on
an L2TPv3 IP encapulation socket.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>