When swapping out, we will split multi-order pages both in order to
move them to the swap-cache and to be able to return memory to the
swap cache as soon as possible on a page-by-page basis.
Reduce the page max order to the system PMD size, as we can then be nicer
to the system and avoid splitting gigantic pages.

Looking forward to when we might be able to swap out PMD size folios
without splitting, this will also be a benefit.

v2:
- Include all orders up to the PMD size (Christian König)
v3:
- Avoid compilation errors for architectures with special PFN_SHIFTs
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230404200650.11043-3-thomas.hellstrom@linux.intel.com