RTM_DELROUTE not sent anymore when deleting (last) nexthop of routes in 6.1

[Jonas Gorski <jonas.gorski@gmail.com>]

Hello,

when an IPv4 route gets removed because its nexthop was deleted, the
kernel does not send a RTM_DELROUTE netlink notifications anymore in
6.1. A bisect lead me to 61b91eb33a69 ("ipv4: Handle attempt to delete
multipath route when fib_info contains an nh reference"), and
reverting it makes it work again.

It can be reproduced by doing the following and listening to netlink
(e.g. via ip monitor)

ip a a 172.16.1.1/24 dev veth1
ip nexthop add id 100 via 172.16.1.2 dev veth1
ip route add 172.16.101.0/24 nhid 100
ip nexthop del id 100

where the nexthop del will trigger a RTM_DELNEXTHOP message, but no
RTM_DELROUTE, but the route is gone afterwards anyways.

Doing the same thing with IPv6 still works as expected

ip a a 2001:db8:91::1/64 dev veth1
ip nexthop add id 100 via 2001:db8:91::2 dev veth1
ip route add 2001:db8:101::/64 nhid 100
ip nexthop del id 100

Here the kernel will send out both the RTM_DELNEXTHOP and the
RTM_DELROUTE netlink messages.

Unfortunately my net-foo is not good enough to propose a fix.

Best regards
Jonas