net: Fix memory leaks in lltable_calc_llheader() error paths

Also convert raw epoch_call() calls to lltable_free_entry() calls, no
functional change intended. There's no need to asynchronously free the
LLEs in that case to begin with, but we might as well use the lltable
interfaces consistently.

Noticed by code inspection; I believe lltable_calc_llheader() failures
do not generally happen in practice.

Reviewed by: bz
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34832