Convert much of the page cache to use folios This patchset stops just short of actually enabling large folios. It converts everything that I noticed needs to be converted, but there may still be places I've overlooked which still have page size assumptions. The big change here is using large entries in the page cache XArray instead of many small entries. That only affects shmem for now, but it's a pretty big change for shmem since it changes where memory needs to be allocated (at split time instead of insertion).