f2fs: cover number of dirty node pages under node_write lock

This number is referenced by checkpoint under node_write lock.
Reviewed-by: NChao Yu <chao2.yu@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

@@ -1323,23 +1323,24 @@ static int f2fs_write_node_page(struct page *page,
 	nid = nid_of_node(page);
 	f2fs_bug_on(sbi, page->index != nid);
 
+	if (wbc->for_reclaim) {
+		if (!down_read_trylock(&sbi->node_write))
+			goto redirty_out;
+	} else {
+		down_read(&sbi->node_write);
+	}
+
 	get_node_info(sbi, nid, &ni);
 
 	/* This page is already truncated */
 	if (unlikely(ni.blk_addr == NULL_ADDR)) {
 		ClearPageUptodate(page);
 		dec_page_count(sbi, F2FS_DIRTY_NODES);
+		up_read(&sbi->node_write);
 		unlock_page(page);
 		return 0;
 	}
 
-	if (wbc->for_reclaim) {
-		if (!down_read_trylock(&sbi->node_write))
-			goto redirty_out;
-	} else {
-		down_read(&sbi->node_write);
-	}
-
 	set_page_writeback(page);
 	fio.blk_addr = ni.blk_addr;
 	write_node_page(nid, &fio);