Use completely static-sized buffers

(cherry picked from commit 49f90025)
(cherry picked from commit 4007a9d0)
7 jobs for 180-fix-cmsgbuf-usage-v9_12-v9_11 in 5 minutes and 53 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Precheck
passed #20696
amd64 docker linux
precheck:debian:sid:amd64

00:00:11

 
  Build
passed #20697
amd64 docker linux
build:debian:sid:amd64

00:01:26

passed #20698
i386 docker linux
build:debian:sid:i386

00:01:30

 
  Test
passed #20701
amd64 docker linux
systemtest:debian:sid:amd64

00:03:57

passed #20702
i386 docker linux
systemtest:debian:sid:i386

00:04:10

passed #20699
amd64 docker linux
unittest:debian:sid:amd64

00:00:47

passed #20700
i386 docker linux
unittest:debian:sid:i386

00:00:45