f2fs: fix to check available space of CP area correctly in update_ckpt_flags()

Otherwise, nat_bit area may be persisted across boundary of CP area during
nat_bit rebuilding.

Fixes: 94c821fb ("f2fs: rebuild nat_bits during umount")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/checkpoint.c

@@ -1302,8 +1302,8 @@ static void update_ckpt_flags(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 	unsigned long flags;
 
 	if (cpc->reason & CP_UMOUNT) {
-		if (le32_to_cpu(ckpt->cp_pack_total_block_count) >
-			sbi->blocks_per_seg - NM_I(sbi)->nat_bits_blocks) {
+		if (le32_to_cpu(ckpt->cp_pack_total_block_count) +
+			NM_I(sbi)->nat_bits_blocks > sbi->blocks_per_seg) {
 			clear_ckpt_flags(sbi, CP_NAT_BITS_FLAG);
 			f2fs_notice(sbi, "Disable nat_bits due to no space");
 		} else if (!is_set_ckpt_flags(sbi, CP_NAT_BITS_FLAG) &&