bonding: add new option ns_ip6_target

This patch add a new bonding option ns_ip6_target, which correspond
to the arp_ip_target. With this we set IPv6 targets and send IPv6 NS
request to determine the health of the link.

For other related options like the validation, we still use
arp_validate, and will change to ns_validate later.

Note: the sysfs configuration support was removed based on
https://lore.kernel.org/netdev/8863.1645071997@famine



Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>