vrrp: allow a VRRP instance to track multiple interfaces

Commit a0e88035 - "vrrp: use instance fault flags instead of a counter"
made an	incorrect assumption that a VRRP instance would	only track one
interface. This	could never be the case	for VMACs, and need not be the
case for not-VMAC instances.

This commit now allows VRRP instances to track any number of interfaces.

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