khugepaged: use a folio throughout collapse_file()

Pull folios from the page cache instead of pages.  Half of this work had
been done already, but we were still operating on pages for a large chunk
of this function.  There is no attempt in this patch to handle large
folios that are smaller than a THP; that will have to wait for a future
patch.

[willy@infradead.org: the unlikely() is embedded in IS_ERR()]
  Link: https://lkml.kernel.org/r/ZhIWX8K0E2tSyMSr@casper.infradead.org
Link: https://lkml.kernel.org/r/20240403171838.1445826-7-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>