[PATCH][F 0/1][E/D/B 0/5] Reapply RAID0 layout support

[dann frazier]

BugLink: https://bugs.launchpad.net/bugs/1850540

This reapplies the patch we reverted due to LP: #1849682 to Bionic, Disco &
Eoan. Now that the mdadm piece is ready (staged in -proposed) and we've
improved the documentation (see patch 5/5), I think it is ready to ship.

That patch required some minor porting as it modifies code that was
also touched by the "broken" state patches we've applied in the interim.
In addition, 3 upstream fixes to the initial patch are included -
these are clean cherry picks from upstream (2 from stable, and 1 from
mainline that probably should go to stable as well).

An additional SAUCE patch is included for those releases, as well as Focal
(Focal already has the others).

I'll submit a separate series for each release in-reply-to this mail.

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2.24.1