Plug nexthop group refcount leak.
In case with batch route delete via rib_walk_del(), when
some paths from the multipath route gets deleted, old
multipath group were not freed.

PR: 254496
Reported by: Zhenlei Huang <zlei.huang@gmail.com>
Approved by: re (gjb)

(cherry picked from commit 47c00a9835926e96e562c67fa28e4432e99d9c56)