[$] Rethinking multi-grain timestamps

One of the significant features added to the mainline kernel during the 6.6merge window was multi-grain timestamps, which allow the kernel toselectively store file modification times with higher resolution withouthurting performance. Unfortunately, this feature also caused somesurprising regressions, and was quickly ushered back out of the kernel as aresult. It is instructive to look at how this feature went wrong, and howthe developers involved plan to move forward from here.