mm: swap: Update get_swap_pages() to take folio order

We are about to allow swap storage of any mTHP size. To prepare for
that, let's change get_swap_pages() to take a folio order parameter
instead of nr_pages. This makes the interface self-documenting; a
power-of-2 number of pages must be provided. We will also need the order
internally so this simplifies accessing it.

Reviewed-by: "Huang, Ying" <ying.huang@intel.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>