net/l2tp: fix warning in l2tp_exit_net found by syzbot

In the Linux kernel, the following vulnerability has been resolved:

net/l2tp: fix warning in l2tp_exit_net found by syzbot

In l2tp's net exit handler, we check that an IDR is empty before

destroying it:

WARN_ON_ONCE(!idr_is_empty(&pn->l2tp_tunnel_idr));

idr_destroy(&pn->l2tp_tunnel_idr);

By forcing memory allocation failures in idr_alloc_32, syzbot is able

to provoke a condition where idr_is_empty returns false despite there

being no items in the IDR. This turns out to be because the radix tree

of the IDR contains only internal radix-tree nodes and it is this that

causes idr_is_empty to return false. The internal nodes are cleaned by

idr_destroy.

Use idr_for_each to check that the IDR is empty instead of

idr_is_empty to avoid the problem.