pagevec: add ability to iterate a queue

Add a loop counter inside the folio_batch to let us iterate from 0-nr
instead of decrementing nr and treating the batch as a stack.  It would
generate some very weird and suboptimal I/O patterns for page writeback to
iterate over the batch as a stack.

Link: https://lkml.kernel.org/r/20240215063649.2164017-11-hch@lst.deSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Dave Chinner <dchinner@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/pagevec.h

@@ -27,6 +27,7 @@ struct folio;
  */
 struct folio_batch {
 	unsigned char nr;
+	unsigned char i;
 	bool percpu_pvec_drained;
 	struct folio *folios[PAGEVEC_SIZE];
 };
@@ -40,12 +41,14 @@ struct folio_batch {
 static inline void folio_batch_init(struct folio_batch *fbatch)
 {
 	fbatch->nr = 0;
+	fbatch->i = 0;
 	fbatch->percpu_pvec_drained = false;
 }
 
 static inline void folio_batch_reinit(struct folio_batch *fbatch)
 {
 	fbatch->nr = 0;
+	fbatch->i = 0;
 }
 
 static inline unsigned int folio_batch_count(struct folio_batch *fbatch)
@@ -75,6 +78,21 @@ static inline unsigned folio_batch_add(struct folio_batch *fbatch,
 	return folio_batch_space(fbatch);
 }
 
+/**
+ * folio_batch_next - Return the next folio to process.
+ * @fbatch: The folio batch being processed.
+ *
+ * Use this function to implement a queue of folios.
+ *
+ * Return: The next folio in the queue, or NULL if the queue is empty.
+ */
+static inline struct folio *folio_batch_next(struct folio_batch *fbatch)
+{
+	if (fbatch->i == fbatch->nr)
+		return NULL;
+	return fbatch->folios[fbatch->i++];
+}
+
 void __folio_batch_release(struct folio_batch *pvec);
 
 static inline void folio_batch_release(struct folio_batch *fbatch)