f2fs: Convert __read_io_type() to take a folio

Remove the last call to page_file_mapping() as both callers can now pass
in a folio.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>