btrfs: page to folio conversion in btrfs_truncate_block()

Convert use of struct page to struct folio inside btrfs_truncate_block().
The only page based function is set_page_extent_mapped(). All other
functions have folio equivalents.

Had to use __filemap_get_folio() because filemap_grab_folio() does not
allow passing allocation mask as a parameter.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>