Zero struct weightened_nhop fields in nhgrp_get_addition_group().

struct weightened_nhop has spare 32bit between the fields due to
the alignment (on amd64).
Not zeroing these spare bits results in duplicating nhop groups
in the kernel due to the way how comparison works.

Approved by: re (gjb)

(cherry picked from commit 6f43c72b472ee78e04f1ebd347ca0ae7787ee876)