net: bonding: add broadcast_neighbor option for 802.3ad

Stacking technology is a type of technology used to expand ports on
Ethernet switches. It is widely used as a common access method in
large-scale Internet data center architectures. Years of practice
have proved that stacking technology has advantages and disadvantages
in high-reliability network architecture scenarios. For instance,
in stacking networking arch, conventional switch system upgrades
require multiple stacked devices to restart at the same time.
Therefore, it is inevitable that the business will be interrupted
for a while. It is for this reason that "no-stacking" in data centers
has become a trend. Additionally, when the stacking link connecting
the switches fails or is abnormal, the stack will split. Although it is
not common, it still happens in actual operation. The problem is that
after the split, it is equivalent to two switches with the same
configuration appearing in the network, causing network configuration
conflicts and ultimately interrupting the services carried by the
stacking system.

To improve network stability, "non-stacking" solutions have been
increasingly adopted, particularly by public cloud providers and
tech companies like Alibaba, Tencent, and Didi. "non-stacking" is
a method of mimicing switch stacking that convinces a LACP peer,
bonding in this case, connected to a set of "non-stacked" switches
that all of its ports are connected to a single switch
(i.e., LACP aggregator), as if those switches were stacked. This
enables the LACP peer's ports to aggregate together, and requires
(a) special switch configuration, described in the linked article,
and (b) modifications to the bonding 802.3ad (LACP) mode to send
all ARP/ND packets across all ports of the active aggregator.

Note that, with multiple aggregators, the current broadcast mode
logic will send only packets to the selected aggregator(s).

 +-----------+   +-----------+
 |  switch1  |   |  switch2  |
 +-----------+   +-----------+
         ^           ^
         |           |
      +-----------------+
      |   bond4 lacp    |
      +-----------------+
         |           |
         | NIC1      | NIC2
      +-----------------+
      |     server      |
      +-----------------+

- https://www.ruijie.com/fr-fr/support/tech-gallery/de-stack-data-center-network-architecture/

Cc: Jay Vosburgh <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Jakub Kicinski <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: Simon Horman <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Andrew Lunn <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Nikolay Aleksandrov <[email protected]>
Signed-off-by: Tonghao Zhang <[email protected]>
Signed-off-by: Zengbing Tu <[email protected]>
Link: https://patch.msgid.link/84d0a044514157bb856a10b6d03a1028c4883561.1751031306.git.tonghao@bamaicloud.com
Signed-off-by: Paolo Abeni <[email protected]>

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