f2fs: fix not to allocate max_nid

The build_free_nid should not add free nids over nm_i->max_nid.
But, there was a hole that invalid free nid was added by the following scenario.

Let's suppose nm_i->max_nid = 150 and the last NAT page has 100 ~ 200 nids.

build_free_nids
  - get_current_nat_page loads the last NAT page
  - scan_nat_page can add 100 ~ 200 nids
    -> Bug here!
So, when scanning an NAT page, we should check each candidate whether it is
over max_nid or not.
Reviewed-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/node.c

@@ -1268,6 +1268,8 @@ static int scan_nat_page(struct f2fs_nm_info *nm_i,
 	i = start_nid % NAT_ENTRY_PER_BLOCK;
 
 	for (; i < NAT_ENTRY_PER_BLOCK; i++, start_nid++) {
+		if (start_nid >= nm_i->max_nid)
+			break;
 		blk_addr  = le32_to_cpu(nat_blk->entries[i].block_addr);
 		BUG_ON(blk_addr == NEW_ADDR);
 		if (blk_addr == NULL_ADDR)