vrrpd: try to bind interfaces on if_down notifications

vrrpd: try to bind interfaces on if_down notifications

Normally we only opportunistically try to bind interfaces to VRRP
instances upon getting if_add and if_up notifications; now that Zebra
sends if_down notifications when interfaces change while they are down,
we should try to bind when we get those as well.

This solves a bug where VRRP would not bind and activate virtual routers
to valid interfaces because their MACs were changed to VRRP macs while
the interface was down.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>