mm: Add folio_estimated_mapcount()

folio_estimated_mapcount() takes in a folio and calls page_mapcount() on
the first page of that folio.

This is necessary for folio conversions where we only care about either the
entire_mapcount of a large folio, or the mapcount of a not large folio.

This is in contrast to folio_mapcount() which calculates the total
number of the times a folio and its subpages are mapped.

Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>

include/linux/mm.h

@@ -913,6 +913,11 @@ static inline int page_mapcount(struct page *page)
 	return mapcount;
 }
 
+static inline int folio_estimated_mapcount(struct folio *folio)
+{
+	return page_mapcount(folio_page(folio, 0));
+}
+
 int folio_total_mapcount(struct folio *folio);
 
 /**