f2fs: should check the node page was truncated first

Currently, f2fs doesn't reclaim any node pages.
However, if we found that a node page was truncated by checking its block
address with zero during f2fs_write_node_page, we should not skip that node
page and return zero to reclaim it.
Reviewed-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/node.c

@@ -1097,13 +1097,6 @@ static int f2fs_write_node_page(struct page *page,
 	block_t new_addr;
 	struct node_info ni;
 
-	if (wbc->for_reclaim) {
-		dec_page_count(sbi, F2FS_DIRTY_NODES);
-		wbc->pages_skipped++;
-		set_page_dirty(page);
-		return AOP_WRITEPAGE_ACTIVATE;
-	}
-
 	wait_on_page_writeback(page);
 
 	mutex_lock_op(sbi, NODE_WRITE);
@@ -1118,6 +1111,14 @@ static int f2fs_write_node_page(struct page *page,
 	if (ni.blk_addr == NULL_ADDR)
 		goto out;
 
+	if (wbc->for_reclaim) {
+		dec_page_count(sbi, F2FS_DIRTY_NODES);
+		wbc->pages_skipped++;
+		set_page_dirty(page);
+		mutex_unlock_op(sbi, NODE_WRITE);
+		return AOP_WRITEPAGE_ACTIVATE;
+	}
+
 	set_page_writeback(page);
 
 	/* insert node offset */