net: mpls: Fix notifications when deleting a device

stable inclusion
from stable-v5.10.84
commit fd52e1f8c09344b86214cbbdb35d7ff86c8d190a
bugzilla: 186030 https://gitee.com/openeuler/kernel/issues/I4QV2F

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fd52e1f8c09344b86214cbbdb35d7ff86c8d190a



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commit 7d4741ea upstream.

There are various problems related to netlink notifications for mpls route
changes in response to interfaces being deleted:
* delete interface of only nexthop
	DELROUTE notification is missing RTA_OIF attribute
* delete interface of non-last nexthop
	NEWROUTE notification is missing entirely
* delete interface of last nexthop
	DELROUTE notification is missing nexthop

All of these problems stem from the fact that existing routes are modified
in-place before sending a notification. Restructure mpls_ifdown() to avoid
changing the route in the DELROUTE cases and to create a copy in the
NEWROUTE case.

Fixes: f8efb73c ("mpls: multipath route support")
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>