Merge branch 'ipv6-fix-hangup-on-device-removal'

Paolo Abeni says:

====================
ipv6: fix hangup on device removal

This addresses the infamous unregister_netdevice splat in net selftests;
the actual fix is carried by the first patch, while the 2nd one
addresses a related problem in the relevant test that was patially
hiding the problem.

Targeting net-next as the issue is quite old and I feel a little lost
in the fib info/nh jungle.
====================

Link: https://patch.msgid.link/cover.1730828007.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>