f2fs: use nm_i->next_scan_nid as default for next_free_nid

Now, if there is no free nid in nm_i->free_nid_list, 0 may be saved
into next_free_nid of checkpoint, this may cause useless scanning for
next mount.  nm_i->next_scan_nid should be a better default value than
0.
Signed-off-by: Huang, Ying <ying.huang@intel.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/checkpoint.c

@@ -808,7 +808,8 @@ static void do_checkpoint(struct f2fs_sb_info *sbi, bool is_umount)
 {
 	struct f2fs_checkpoint *ckpt = F2FS_CKPT(sbi);
 	struct curseg_info *curseg = CURSEG_I(sbi, CURSEG_WARM_NODE);
-	nid_t last_nid = 0;
+	struct f2fs_nm_info *nm_i = NM_I(sbi);
+	nid_t last_nid = nm_i->next_scan_nid;
 	block_t start_blk;
 	struct page *cp_page;
 	unsigned int data_sum_blocks, orphan_blocks;