[lltable] Unify datapath feedback mechamism.

Use newly-create llentry_request_feedback(),
llentry_mark_used() and llentry_get_hittime() to
request datapatch usage check and fetch the results
in the same fashion both in IPv4 and IPv6.

While here, simplify llentry_provide_feedback() wrapper
by eliminating 1 condition check.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D31390