Building feature/buster in RAM sometimes fails: mksquashfs reaped by OOM killer

For context, initial analysis, and test results, see #16177 (closed).

Next steps:

1. Verify that squashfs-tools 4.3-3.0tails4 does honor -mem consistently (I think I did already but let’s be sure).
2. Find out why squashfs-tools 4.3-11 does not honor -mem consistently, e.g.:
   1. Try to reproduce with pristine 4.3-3 changes
   2. Try reverting the 4.3-3..4.3-11 changes
   3. Try applying the patchset collected by lynxis on top of 4.3-11
3. Report the bug, ideally with a patch attached.

Related issues

• Related to #16177 (closed)
• Related to #16637 (closed)
• Blocks #16209

Original created by @intrigeri on 16457 (Redmine)