lib, zebra: Check for not being a blackhole route

In zebra_interface_nhg_reinstall zebra is checking that the
nhg is a singleton and not a blackhole nhg.  This was originally
done with checking that the nexthop is a NEXTHOP_TYPE_IFINDEX,
NEXTHOP_TYPE_IPV4_IFINDEX and NEXTHOP_TYPE_IPV6_IFINDEX.  This
was excluding NEXTHOP_TYPE_IPV4 and NEXTHOP_TYPE_IPV6.  These
were both possible to be received and maintained from the upper
level protocol for when a route is being recursively resolved.
If we have gotten to this point in zebra_interface_nhg_reinstall
the nexthop group has already been installed at least once
and we *know* that it is actually a valid nexthop.  What the
test is really trying to do is ensure that we are not reinstalling
a blackhole nexthop group( Which is not possible to even be
here by the way, but safety first! ).  So let's change
to test for that instead.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>

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