ipv6: release nexthop on device removal

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

ipv6: release nexthop on device removal

The CI is hitting some aperiodic hangup at device removal time in the

pmtu.sh self-test:

unregister_netdevice: waiting for veth_A-R1 to become free. Usage count = 6

ref_tracker: veth_A-R1@ffff888013df15d8 has 1/5 users at

dst_init+0x84/0x4a0

dst_alloc+0x97/0x150

ip6_dst_alloc+0x23/0x90

ip6_rt_pcpu_alloc+0x1e6/0x520

ip6_pol_route+0x56f/0x840

fib6_rule_lookup+0x334/0x630

ip6_route_output_flags+0x259/0x480

ip6_dst_lookup_tail.constprop.0+0x5c2/0x940

ip6_dst_lookup_flow+0x88/0x190

udp_tunnel6_dst_lookup+0x2a7/0x4c0

vxlan_xmit_one+0xbde/0x4a50 [vxlan]

vxlan_xmit+0x9ad/0xf20 [vxlan]

dev_hard_start_xmit+0x10e/0x360

__dev_queue_xmit+0xf95/0x18c0

arp_solicit+0x4a2/0xe00

neigh_probe+0xaa/0xf0

While the first suspect is the dst_cache, explicitly tracking the dst

owing the last device reference via probes proved such dst is held by

the nexthop in the originating fib6_info.

Similar to commit f5b51fe804ec ("ipv6: route: purge exception on

removal"), we need to explicitly release the originating fib info when

disconnecting a to-be-removed device from a live ipv6 dst: move the

fib6_info cleanup into ip6_dst_ifdown().

Tested running:

./pmtu.sh cleanup_ipv6_exception

in a tight loop for more than 400 iterations with no spat, running an

unpatched kernel I observed a splat every ~10 iterations.