[$] Rethinking multi-grain timestamps

One of the significant features added to the mainline kernel during the 6.6
merge window was multi-grain timestamps, which allow the kernel to
selectively store file modification times with higher resolution without
hurting performance. Unfortunately, this feature also caused some
surprising regressions, and was quickly ushered back out of the kernel as a
result. It is instructive to look at how this feature went wrong, and how
the developers involved plan to move forward from here.