fs/mpage: use blocks_per_folio instead of blocks_per_page

fs/mpage: use blocks_per_folio instead of blocks_per_page

Convert mpage to folios and adjust accounting for the number of blocks
within a folio instead of a single page. This also adjusts the number
of pages we should process to be the size of the folio to ensure we
always read a full folio.

Note that the page cache code already ensures do_mpage_readpage() will
work with folios respecting the address space min order, this ensures
that so long as folio_size() is used for our requirements mpage will
also now be able to process block sizes larger than the page size.

Originally-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20250221223823.1680616-5-mcgrof@kernel.org
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>