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).
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