zebra: Move id counter into zebra_nhg_find()

zebra: Move id counter into zebra_nhg_find()

Move the id counter further up into zebra_nhg_find() so that
it is still incremented if we receive a duplicate that never
would get allocated. The kernel will still use the dup, so we
have to account for that in our id counter.

Also, if we don't create a new entry, reset the id back to where
it was when zebra_nhg_find() was called.

Signed-off-by: Stephen Worley <sworley@cumulusnetworks.com>