if_tuntap: fix NOIP build

Note: this removes one TUNDEBUG() for the sake of not having one more
ifdefed variable declaration and for the overall code brevity. The call
from tuntap into LRO can be so easily traced with dtrace(1) that an
80-ish printf(9)-based debugging can be omitted.

Fixes: 99c79cab422705f92f05a2924a29bdf823372ebf
(cherry picked from commit 5b0010b4678d778967a5a82fb38507e46a071e38)