zebra: remove default vrf output for kernel vrf ipv6 blackhole default

zebra: remove default vrf output for kernel vrf ipv6 blackhole default

It was reported that "show ipv6 route vrf <vrfname>", "show ipv6 route
vrf <vrfname> ::/0 " or "show ipv6 route vrf <vrfname> json" all
displayed that the nexthop was in the default vrf.  This was because
the kernel netlink messages would supply the RTA_OIF of the loopback
interface for the kernel-created default route for the vrf, where ipv4
did not supply any RTA_OIF.  This fix suppresses the display if the
nexthop and route entry are in different vrfs and the nexthop is
NEXTHOP_TYPE_BLACKHOLE.

Ticket: CM-21722
Signed-off-by: Don Slice <dslice@cumulusnetworks.com>