bonding: guard ns_targets by CONFIG_IPV6

bonding: guard ns_targets by CONFIG_IPV6

Guard ns_targets in struct bond_params by CONFIG_IPV6, which could save
256 bytes if IPv6 not configed. Also add this protection for function
bond_is_ip6_target_ok() and bond_get_targets_ip6().

Remove the IS_ENABLED() check for bond_opts[] as this will make
BOND_OPT_NS_TARGETS uninitialized if CONFIG_IPV6 not enabled. Add
a dummy bond_option_ns_ip6_targets_set() for this situation.

Fixes: 4e24be018eb9 ("bonding: add new parameter ns_targets")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Jonathan Toppins <jtoppins@redhat.com>
Link: https://lore.kernel.org/r/20220531063727.224043-1-liuhangbin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>