net/l2tp: fix warning in l2tp_exit_net found by syzbot

[ Upstream commit 5d066766c5f1252f98ff859265bcd1a5b52ac46c ]

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.

Reported-by:  <syzbot+332fe1e67018625f63c9@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=332fe1e67018625f63c9


Fixes: 73d33bd0 ("l2tp: avoid using drain_workqueue in l2tp_pre_exit_net")
Signed-off-by: James Chapman <jchapman@katalix.com>
Link: https://patch.msgid.link/20241118140411.1582555-1-jchapman@katalix.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>