ext4: fix a double free in ext4_register_li_request

In ext4_register_li_request, we malloc a ext4_li_request and
inserts it into ext4_li_info->li_request_list. In case of any
error later, we free it in the end.  But if we have some error
in ext4_run_lazyinit_thread, the whole li_request_list will be
dropped and freed in it. So we will double free this ext4_li_request.

This patch just sets elr to NULL after it is inserted to the list
so that the latter kfree won't double free it.
Signed-off-by: NTao Ma <boyu.mt@taobao.com>
Reviewed-by: NLukas Czerner <lczerner@redhat.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/super.c

@@ -2975,6 +2975,12 @@ static int ext4_register_li_request(struct super_block *sb,
 	mutex_unlock(&ext4_li_info->li_list_mtx);
 
 	sbi->s_li_request = elr;
+	/*
+	 * set elr to NULL here since it has been inserted to
+	 * the request_list and the removal and free of it is
+	 * handled by ext4_clear_request_list from now on.
+	 */
+	elr = NULL;
 
 	if (!(ext4_li_info->li_state & EXT4_LAZYINIT_RUNNING)) {
 		ret = ext4_run_lazyinit_thread();