f2fs: fix the calculation of max_nids

Total nids that f2fs can use should not include 0, nid for node inode, and nid
for meta inode.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/node.c

@@ -1811,7 +1811,9 @@ static int init_node_manager(struct f2fs_sb_info *sbi)
 	/* segment_count_nat includes pair segment so divide to 2. */
 	nat_segs = le32_to_cpu(sb_raw->segment_count_nat) >> 1;
 	nat_blocks = nat_segs << le32_to_cpu(sb_raw->log_blocks_per_seg);
-	nm_i->max_nid = NAT_ENTRY_PER_BLOCK * nat_blocks;
+
+	/* not used nids: 0, node, meta, (and root counted as valid node) */
+	nm_i->max_nid = NAT_ENTRY_PER_BLOCK * nat_blocks - 3;
 	nm_i->fcnt = 0;
 	nm_i->nat_cnt = 0;