update blacklist atomically with `ipset swap`

Thanks for the script, I'm using it on my OpenWRT router. I made this patch to create a temporary ipset with the updated IP list and swap with the actual blacklist so the blacklist is never empty (instead of flushing the blacklist and then generating the new one). It takes almost a minute to create the new blacklist on my router, and this avoids a small amount of downtime.