l2tp: hold tunnel in pppol2tp_connect()

Use l2tp_tunnel_get() in pppol2tp_connect() to ensure the tunnel isn't
going to disappear while processing the rest of the function.

Fixes: fd558d18 ("l2tp: Split pppol2tp patch into separate l2tp and ppp parts")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>