l2tp: initialise session's refcount before making it reachable

l2tp: initialise session's refcount before making it reachable

commit 9ee369a405c57613d7c83a3967780c3e30c52ecc upstream.

Sessions must be fully initialised before calling
l2tp_session_add_to_tunnel(). Otherwise, there's a short time frame
where partially initialised sessions can be accessed by external users.

Fixes: dbdbc73b4478 ("l2tp: fix duplicate session creation")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: keep using l2tp_session_inc_refcount()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>