vxlan: use exit_batch_rtnl() method

vxlan: use exit_batch_rtnl() method

exit_batch_rtnl() is called while RTNL is held,
and devices to be unregistered can be queued in the dev_kill_list.

This saves one rtnl_lock()/rtnl_unlock() pair per netns
and one unregister_netdevice_many() call.

v4: (Paolo feedback : https://netdev-3.bots.linux.dev/vmksft-net/results/453141/17-udpgro-fwd-sh/stdout )
  - Changed vxlan_destroy_tunnels() to use vxlan_dellink()
    instead of unregister_netdevice_queue to propely remove
    devices from vn->vxlan_list.
  - vxlan_destroy_tunnels() can simply iterate one list (vn->vxlan_list)
    to find all devices in the most efficient way.
  - Moved sanity checks in a separate vxlan_exit_net() method.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Antoine Tenart <atenart@kernel.org>
Link: https://lore.kernel.org/r/20240206144313.2050392-10-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>