net: ipv6: Add support for RTM_DELNETCONF

Send RTM_DELNETCONF notifications when a device is deleted. The message only
needs the device index, so modify inet6_netconf_fill_devconf to skip devconf
references if it is NULL.

Allows a userspace cache to remove entries as devices are deleted.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>