MFC r288297 (userland part).
If ever MFC is done for the new lltable code, this change will miminise
ABI breakage.
rtsock requests for deleting interface address lles started to return EPERM
instead of old "ignore-and-return 0" in r287789. This broke arp -da / ndp -cn behavior (they exit on rtsock command failure). Fix this by translating LLE_IFADDR to RTM_PINNED flag, passing it to userland and making arp/ndp ignore these entries in batched delete.