Tolerate RTM_DELADDR messages from deleted interfaces.

When an interface is deleted (which can happen, for example, to
the tun interfaces used for VPN and 464xlat), the kernel sends
RTM_DELADDR messages for all its IP addresses.

Currently we ignore these because we cannot fetch the interface
name. Instead, pass them up with an empty interface name so that
NetlinkHandler can do something useful for them, such as destroy
connections on the IP addresses that are being deleted.

Bug: 26976388
Change-Id: I2ced7e389228f9d665d0a87d97bd3452c8c0c98b