zebra: Fix leaked nhe (backport #17809)

[mergify]

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.

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This is an automatic backport of pull request #17809 done by Mergify.

[mergify]

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