netinet6: Fix mbuf leak in NDP

Mbufs leak when manually removing incomplete NDP records with pending packet via ndp -d.
It happens because lltable_drop_entry_queue() rely on la_numheld
counter when dropping NDP entries (lles). It turned out NDP code never
increased la_numheld, so the actual free never happened.

Fix the issue by introducing unified lltable_append_entry_queue(),
common for both ARP and NDP code, properly addressing packet queue
maintenance.

Reviewed By: melifaro
Differential Revision: https://reviews.freebsd.org/D35365
MFC after: 2 weeks

(cherry picked from commit d18b4bec98f1cf3c51103a22c0c041e6238c44c7)