dax: use folios more widely within DAX

Convert from pfn to folio instead of page and use those folios throughout
to avoid accesses to page->index and page->mapping.

Link: https://lkml.kernel.org/r/20241216155408.8102-2-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jane Chu <jane.chu@oracle.com>
Cc: Dan Willaims <dan.j.williams@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Alistair Popple <apopple@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>