fs: Change readpage to take a folio

fs: Change readpage to take a folio

We track the uptodate state on the entire folio, not individual
pages.  That means the readpage should be passed a folio and told
to update the entire folio.  Filesystems will not have multi-page
folios created for them until they indicate support for them by
setting the FS_MULTI_PAGE_FOLIO flag.  Until they do, they can
assume that the folio being passed in contains a single page.

Also convert filler_t to take a folio as these two are tightly
intertwined.

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