mpage: Convert do_mpage_readpage() to use a folio

Pass in a folio from mpage_readahead().  Also convert map_buffer_to_page()
to map_buffer_to_folio().  There's still no support for large folios here;
there are numerous places which depend on the folio being PAGE_SIZE.
The VM_BUG_ON prevents anyone from thinking that it will work.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>