f2fs: check the level before calling get_nid function

The caller of get_nid should be careful not to put lower value than
NODE_DIR1_BLOCK in case of level is zero.
Signed-off-by: Changman Lee <cm224.lee@samsung.com>
Reviewed-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/node.c

@@ -403,6 +403,7 @@ int get_dnode_of_data(struct dnode_of_data *dn, pgoff_t index, int mode)
 		return PTR_ERR(npage[0]);
 
 	parent = npage[0];
+	if (level != 0)
 		nids[1] = get_nid(parent, offset[0], true);
 	dn->inode_page = npage[0];
 	dn->inode_page_locked = true;