exfat: use updated exfat_chain directly during renaming

In order for a file to access its own directory entry set,
exfat_inode_info(ei) has two copied values. One is ei->dir, which is
a snapshot of exfat_chain of the parent directory, and the other is
ei->entry, which is the offset of the start of the directory entry set
in the parent directory.

Since the parent directory can be updated after the snapshot point,
it should be used only for accessing one's own directory entry set.

However, as of now, during renaming, it could try to traverse or to
allocate clusters via snapshot values, it does not make sense.

This potential problem has been revealed when exfat_update_parent_info()
was removed by commit d8dad25 ("exfat: fix referencing wrong parent
directory information after renaming"). However, I don't think it's good
idea to bring exfat_update_parent_info() back.

Instead, let's use the updated exfat_chain of parent directory diectly.

Fixes: d8dad25 ("exfat: fix referencing wrong parent directory information after renaming")
Reported-by: Wang Yugui <wangyugui@e16-tech.com>
Signed-off-by: Sungjong Seo <sj1557.seo@samsung.com>
Tested-by: Wang Yugui <wangyugui@e16-tech.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

Author: Sungjong Seo

fs/exfat/namei.c

@@ -1198,7 +1198,9 @@ static int __exfat_rename(struct inode *old_parent_inode,
 		return -ENOENT;
 	}
 
-	exfat_chain_dup(&olddir, &ei->dir);
+	exfat_chain_set(&olddir, EXFAT_I(old_parent_inode)->start_clu,
+		EXFAT_B_TO_CLU_ROUND_UP(i_size_read(old_parent_inode), sbi),
+		EXFAT_I(old_parent_inode)->flags);
 	dentry = ei->entry;
 
 	ep = exfat_get_dentry(sb, &olddir, dentry, &old_bh);