ruleset: reduce ksoftirqd load by refering to looopback by numeric id

ruleset: reduce ksoftirqd load by refering to looopback by numeric id

Reduce ksoftirq load by half using more efficient reference to loopback
which always has index equal to one.

Should help a lot with openwrt/openwrt#12914, openwrt/openwrt#12121 and
similar iperf3 cases clamping against 100% CPU usage.

Signed-off-by: Andris PE <neandris@gmail.com>
[fix S-o-b tag, fix commit author, rewrap commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>