vrrp: an RTM_DELLINK netling message doesn't always mean i/f deleted

Removing an interface from a bridge, e.g. "ip link set eth0 nomaster"
causes an RTM_DELLINK netlink message to be sent for eth0, even	though
that interface is not deleted.

It appears that	when an	interface is really deleted, ifi_change is
set to 0xffffffff. In the case where the interface is removed from a
bridge ifi_change is set to 0.

This commit makes an RTM_DELLINK message without ifi_change ==
0xffffffff be treated just as an update to the interface rather
than an interface deletion.

Signed-off-by: Quentin Armitage <quentin@armitage.org.uk>