net: no longer hold RTNL while calling flush_all_backlogs()

flush_all_backlogs() is called from unregister_netdevice_many_notify()
as part of netdevice dismantles.

This is currently called under RTNL, and can last up to 50 ms
on busy hosts.

There is no reason to hold RTNL at this stage, if our caller
is cleanup_net() : netns are no more visible, devices
are in NETREG_UNREGISTERING state and no other thread
could mess our state while RTNL is temporarily released.

In order to provide isolation, this patch provides a separate
'net_todo_list' for cleanup_net().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jesse Brandeburg <jbrandeburg@cloudflare.com>
Link: https://patch.msgid.link/20250114205531.967841-4-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>