exfat: fix double free in delayed_free

The double free could happen in the following path.

exfat_create_upcase_table()
        exfat_create_upcase_table() : return error
        exfat_free_upcase_table() : free ->vol_utbl
        exfat_load_default_upcase_table : return error
     exfat_kill_sb()
           delayed_free()
                  exfat_free_upcase_table() <--------- double free
This patch set ->vol_util as NULL after freeing it.

Reported-by: Jianzhou Zhao <[email protected]>
Signed-off-by: Namjae Jeon <[email protected]>

Author: namjaejeon

fs/exfat/nls.c

@@ -801,4 +801,5 @@ int exfat_create_upcase_table(struct super_block *sb)
 void exfat_free_upcase_table(struct exfat_sb_info *sbi)
 {
 	kvfree(sbi->vol_utbl);
+	sbi->vol_utbl = NULL;
 }