fs: Convert writeback to XArray

A couple of short loops.
Signed-off-by: Matthew Wilcox <willy@infradead.org>

fs/fs-writeback.c

@@ -339,9 +339,9 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
 	struct address_space *mapping = inode->i_mapping;
 	struct bdi_writeback *old_wb = inode->i_wb;
 	struct bdi_writeback *new_wb = isw->new_wb;
-	struct radix_tree_iter iter;
+	XA_STATE(xas, &mapping->i_pages, 0);
+	struct page *page;
 	bool switched = false;
-	void **slot;
 
 	/*
 	 * By the time control reaches here, RCU grace period has passed
@@ -375,26 +375,19 @@ static void inode_switch_wbs_work_fn(struct work_struct *work)
 	 * to possibly dirty pages while PAGECACHE_TAG_WRITEBACK points to
 	 * pages actually under writeback.
 	 */
-	radix_tree_for_each_tagged(slot, &mapping->i_pages, &iter, 0,
-				   PAGECACHE_TAG_DIRTY) {
-		struct page *page = radix_tree_deref_slot_protected(slot,
-						&mapping->i_pages.xa_lock);
-		if (likely(page) && PageDirty(page)) {
+	xas_for_each_marked(&xas, page, ULONG_MAX, PAGECACHE_TAG_DIRTY) {
+		if (PageDirty(page)) {
 			dec_wb_stat(old_wb, WB_RECLAIMABLE);
 			inc_wb_stat(new_wb, WB_RECLAIMABLE);
 		}
 	}
 
-	radix_tree_for_each_tagged(slot, &mapping->i_pages, &iter, 0,
-				   PAGECACHE_TAG_WRITEBACK) {
-		struct page *page = radix_tree_deref_slot_protected(slot,
-						&mapping->i_pages.xa_lock);
-		if (likely(page)) {
+	xas_set(&xas, 0);
+	xas_for_each_marked(&xas, page, ULONG_MAX, PAGECACHE_TAG_WRITEBACK) {
 		WARN_ON_ONCE(!PageWriteback(page));
 		dec_wb_stat(old_wb, WB_WRITEBACK);
 		inc_wb_stat(new_wb, WB_WRITEBACK);
 	}
-	}
 
 	wb_get(new_wb);