scatterlist: add sg_set_folio()

This wrapper for sg_set_page() lets drivers add folios to a scatterlist
more easily.  We could, perhaps, do better by using a different page in
the folio if offset is larger than UINT_MAX, but let's hope we get a
better data structure than this before we need to care about such large
folios.

Link: https://lkml.kernel.org/r/20230621164557.3510324-4-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/scatterlist.h

@@ -141,6 +141,30 @@ static inline void sg_set_page(struct scatterlist *sg, struct page *page,
 	sg->length = len;
 }
 
+/**
+ * sg_set_folio - Set sg entry to point at given folio
+ * @sg:		 SG entry
+ * @folio:	 The folio
+ * @len:	 Length of data
+ * @offset:	 Offset into folio
+ *
+ * Description:
+ *   Use this function to set an sg entry pointing at a folio, never assign
+ *   the folio directly. We encode sg table information in the lower bits
+ *   of the folio pointer. See sg_page() for looking up the page belonging
+ *   to an sg entry.
+ *
+ **/
+static inline void sg_set_folio(struct scatterlist *sg, struct folio *folio,
+			       size_t len, size_t offset)
+{
+	WARN_ON_ONCE(len > UINT_MAX);
+	WARN_ON_ONCE(offset > UINT_MAX);
+	sg_assign_page(sg, &folio->page);
+	sg->offset = offset;
+	sg->length = len;
+}
+
 static inline struct page *sg_page(struct scatterlist *sg)
 {
 #ifdef CONFIG_DEBUG_SG