net: force a list_del() in unregister_netdevice_many()

unregister_netdevice_many() API is error prone and we had too
many bugs because of dangling LIST_HEAD on stacks.

See commit f87e6f47 ("net: dont leave active on stack LIST_HEAD")

In fact, instead of making sure no caller leaves an active list_head,
just force a list_del() in the callee. No one seems to need to access
the list after unregister_netdevice_many()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>