ipv6: Fix signed integer overflow in l2tp_ip6_sendmsg

stable inclusion
from stable-v4.19.249
commit 6c4e3486d21173d60925ef52e512cae727b43d30
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I970CO

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6c4e3486d21173d60925ef52e512cae727b43d30



--------------------------------

[ Upstream commit f638a84a ]

When len >= INT_MAX - transhdrlen, ulen = len + transhdrlen will be
overflow. To fix, we can follow what udpv6 does and subtract the
transhdrlen from the max.

Signed-off-by: Wang Yufen <wangyufen@huawei.com>
Link: https://lore.kernel.org/r/20220607120028.845916-2-wangyufen@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>