bonding: add ns target multicast address to slave device

bonding: add ns target multicast address to slave device

Commit 4598380f9c54 ("bonding: fix ns validation on backup slaves")
tried to resolve the issue where backup slaves couldn't be brought up when
receiving IPv6 Neighbor Solicitation (NS) messages. However, this fix only
worked for drivers that receive all multicast messages, such as the veth
interface.

For standard drivers, the NS multicast message is silently dropped because
the slave device is not a member of the NS target multicast group.

To address this, we need to make the slave device join the NS target
multicast group, ensuring it can receive these IPv6 NS messages to validate
the slave’s status properly.

There are three policies before joining the multicast group:
1. All settings must be under active-backup mode (alb and tlb do not support
   arp_validate), with backup slaves and slaves supporting multicast.
2. We can add or remove multicast groups when arp_validate changes.
3. Other operations, such as enslaving, releasing, or setting NS targets,
   need to be guarded by arp_validate.

Fixes: 4e24be018eb9 ("bonding: add new parameter ns_targets")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>