[$] On-disk format robustness requirements for new filesystems

The "Extendable Read-Only File System" (or "EROFS") was first postedby Gao Xiang in May 2018; it was merged into the staging tree forthe 4.19 release. There has been a steady stream of work on EROFS sincethen, and its author now thinks that it is ready to move out of stagingand join the other official filesystems in the kernel. It would seem,though, that there is one final hurdle that it may have to clear:robustness in the face of a corrupted on-disk filesystem image. Thatraises an interesting question: to what extent do new filesystems have toexhibit a level of robustness that is not met by the filesystems that arecurrently in heavy use?