ln_expire
ln->ln_expire = time_uptime + xtick / hz;
if (ln->ln_expire > time_uptime)
xtick = (ln->ln_expire - time_uptime) * hz;
(((ln)->ln_expire == 0) && ((ln)->ln_state > ND_LLINFO_INCOMPLETE))
nbi->expire = ln->ln_expire ?
time_mono_to_wall(ln->ln_expire) : 0;
nbi->expire = ln->ln_expire ?
time_mono_to_wall(ln->ln_expire) : 0;