l2tp: don't close sessions in l2tp_tunnel_destruct()

Sessions are already removed by the proto ->destroy() handlers, and
since commit f3c66d4e ("l2tp: prevent creation of sessions on terminated tunnels"),
we're guaranteed that no new session can be created afterwards.

Furthermore, l2tp_tunnel_closeall() can sleep when there are sessions
left to close. So we really shouldn't call it in a ->sk_destruct()
handler, as it can be used from atomic context.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>