netlink: cleanup the unnecessary return value check

It's no needed to check the return value of tab since the NULL situation
has been handled already, and the rtnl_msg_handlers[PF_UNSPEC] has been
initialized as non-NULL during the rtnetlink_init().

Signed-off-by: Hans Zhang <zhanghonghui@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>