zebra: Fix leaked nhe

zebra: Fix leaked nhe

During route processing in zebra, Zebra will create a nexthop
group that matches the nexthops passed down from the routing
protocol.  Then Zebra will look to see if it can re-use a
nhe from a previous version of the route entry( say a interface
goes down ).  If Zebra decides to re-use an nhe it was just dropping
the route entry created.  Which led to nexthop group's that had
a refcount of 0 and in some cases these nexthop groups were installed
into the kernel.

Add a bit of code to see if the returned entry is not being used
and it has no reference count and if so, properly dispose of it.

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