zebra: Fix resetting valid flags for NHG dependents (backport #17731)

[mergify]

Upon if_down, we don't reset the valid flag for dependents and unset the INSTALLED flag.

So when its time for the NHG to be deleted (routes dereferenced), zebra deletes it since refcnt goes to 0, but stale NHG remains in kernel.

Ticket :#4200788

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This is an automatic backport of pull request #17731 done by Mergify.

[mergify]

Cherry-pick of 54ec9f3 has failed:

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	both modified:   zebra/zebra_nhg.c

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