mm: add folio_flush_mapping()

This is the folio equivalent of page_mapping_file(), but rename it to make
it clear that it's very different from page_file_mapping(). 
Theoretically, there's nothing flush-only about it, but there are no other
users today, and I doubt there will be; it's almost always more useful to
know the swapfile's mapping or the swapcache's mapping.

Link: https://lkml.kernel.org/r/20230802151406.3735276-5-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/pagemap.h

@@ -389,6 +389,26 @@ static inline struct address_space *folio_file_mapping(struct folio *folio)
 	return folio->mapping;
 }
 
+/**
+ * folio_flush_mapping - Find the file mapping this folio belongs to.
+ * @folio: The folio.
+ *
+ * For folios which are in the page cache, return the mapping that this
+ * page belongs to.  Anonymous folios return NULL, even if they're in
+ * the swap cache.  Other kinds of folio also return NULL.
+ *
+ * This is ONLY used by architecture cache flushing code.  If you aren't
+ * writing cache flushing code, you want either folio_mapping() or
+ * folio_file_mapping().
+ */
+static inline struct address_space *folio_flush_mapping(struct folio *folio)
+{
+	if (unlikely(folio_test_swapcache(folio)))
+		return NULL;
+
+	return folio_mapping(folio);
+}
+
 static inline struct address_space *page_file_mapping(struct page *page)
 {
 	return folio_file_mapping(page_folio(page));
@@ -399,11 +419,7 @@ static inline struct address_space *page_file_mapping(struct page *page)
  */
 static inline struct address_space *page_mapping_file(struct page *page)
 {
-	struct folio *folio = page_folio(page);
-
-	if (unlikely(folio_test_swapcache(folio)))
-		return NULL;
-	return folio_mapping(folio);
+	return folio_flush_mapping(page_folio(page));
 }
 
 /**