bonding: NS target should accept link local address

When setting bond NS target, we use bond_is_ip6_target_ok() to check
if the address valid. The link local address was wrongly rejected in
bond_changelink(), as most time the user just set the ARP/NS target to
gateway, while the IPv6 gateway is always a link local address when user
set up interface via SLAAC.

So remove the link local addr check when setting bond NS target.

Fixes: 129e3c1b ("bonding: add new option ns_ip6_target")
Reported-by: Li Liang <liali@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Jonathan Toppins <jtoppins@redhat.com>
Acked-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>