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>

(cherry picked from commit 66f138563becf12d5c21924f816d2a45c3a1ed7a)