netlink: use nla_get_in_addr and nla_put_in_addr for ipv4 address

Since nla_get_in_addr and nla_put_in_addr were implemented,
so use them appropriately.

Signed-off-by: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>