l2tp: prevent pppol2tp_connect() from creating kernel sockets

If 'fd' is negative, l2tp_tunnel_create() creates a tunnel socket using
the configuration passed in 'tcfg'. Currently, pppol2tp_connect() sets
the relevant fields to zero, tricking l2tp_tunnel_create() into setting
up an unusable kernel socket.

We can't set 'tcfg' with the required fields because there's no way to
get them from the current connect() parameters. So let's restrict
kernel sockets creation to the netlink API, which is the original use
case.

Fixes: 789a4a2c ("l2tp: Add support for static unmanaged L2TPv3 tunnels")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>