vrrp: fix support for VRs in different FIBs

vrrp: fix support for VRs in different FIBs

Type: fix
Fixes: 4b1b13315a3c

When adding or deleting a VR, multicast routes can be added or deleted.
When the first VR is added, a local (dpo-receive) route is added. The
route is deleted when the last VR is deleted.

Perform the check on whether to add or delete the route on a per-FIB
basis. Otherwise, if the route is only added after the first VR is added
without regards to the FIB being used and a second VR is added later on
an interface attached to a different FIB, the necessary route will not be
added to the FIB used by the second interface.

Change-Id: Ib30925ecf45c714cfe3ac6a223754bea918f10e3
Signed-off-by: Matthew Smith <mgsmith@netgate.com>