i40iw: Avoid panic when handling the inetdev event

There is a panic reported that on a system with x722 ethernet, when doing
the operations like:

	# ip link add br0 type bridge
	# ip link set eno1 master br0
	# systemctl restart systemd-networkd

The system will panic "BUG: unable to handle kernel null pointer
dereference at 0000000000000034", with call chain:

	i40iw_inetaddr_event
	notifier_call_chain
	blocking_notifier_call_chain
	notifier_call_chain
	__inet_del_ifa
	inet_rtm_deladdr
	rtnetlink_rcv_msg
	netlink_rcv_skb
	rtnetlink_rcv
	netlink_unicast
	netlink_sendmsg
	sock_sendmsg
	__sys_sendto

It is caused by "local_ipaddr = ntohl(in->ifa_list->ifa_address)", while
the in->ifa_list is NULL.

So add a check for the "in->ifa_list == NULL" case, and skip the ARP
operation accordingly.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>