khugepaged: use a folio throughout hpage_collapse_scan_file()

Replace the use of pages with folios.  Saves a few calls to
compound_head() and removes some uses of obsolete functions.

Link: https://lkml.kernel.org/r/20240403171838.1445826-8-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>