mm: convert page_mapcount() to use folio_entire_mapcount()

Remove a use of head_compound_mapcount().

Link: https://lkml.kernel.org/r/20230111142915.1001531-15-willy@infradead.org


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