Use completely static-sized buffers

(cherry picked from commit 49f90025)
7 jobs for 180-fix-cmsgbuf-usage-v9_12 in 6 minutes and 22 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Precheck
passed #20564
linux docker amd64
precheck:debian:sid:amd64

00:00:12

 
  Build
passed #20565
linux docker amd64
build:debian:sid:amd64

00:01:21

passed #20566
linux docker i386
build:debian:sid:i386

00:01:26

 
  Test
passed #20569
linux docker amd64
systemtest:debian:sid:amd64

00:04:42

passed #20570
linux docker i386
systemtest:debian:sid:i386

00:04:42

passed #20567
linux docker amd64
unittest:debian:sid:amd64

00:00:45

passed #20568
linux docker i386
unittest:debian:sid:i386

00:00:47