folio: Add a function to change the private data attached to a folio

Add a function, folio_change_private(), that will change the private data
attached to a folio, without the need to twiddle the private bit or the
refcount.  It assumes that folio_add_private() has already been called on
the page.
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Tested-by: Jeff Layton <jlayton@kernel.org>
Tested-by: Dominique Martinet <asmadeus@codewreck.org>
Tested-by: kafs-testing@auristor.com
Link: https://lore.kernel.org/r/162981149911.1901565.17776700811659843340.stgit@warthog.procyon.org.uk/
Link: https://lore.kernel.org/r/163005743485.2472992.5100702469503007023.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163584180781.4023316.5037526301198034310.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/163649324326.309189.17817587229450840783.stgit@warthog.procyon.org.uk/ # v4
Link: https://lore.kernel.org/r/163657848531.834781.14269656212269187893.stgit@warthog.procyon.org.uk/ # v5

include/linux/pagemap.h

@@ -279,6 +279,25 @@ static inline void folio_attach_private(struct folio *folio, void *data)
 	folio_set_private(folio);
 }
 
+/**
+ * folio_change_private - Change private data on a folio.
+ * @folio: Folio to change the data on.
+ * @data: Data to set on the folio.
+ *
+ * Change the private data attached to a folio and return the old
+ * data.  The page must previously have had data attached and the data
+ * must be detached before the folio will be freed.
+ *
+ * Return: Data that was previously attached to the folio.
+ */
+static inline void *folio_change_private(struct folio *folio, void *data)
+{
+	void *old = folio_get_private(folio);
+
+	folio->private = data;
+	return old;
+}
+
 /**
  * folio_detach_private - Detach private data from a folio.
  * @folio: Folio to detach data from.