Hello.

kernel: [  149.507103] ================================================
kernel: [  149.507113] [ BUG: lock held when returning to user space! ]
kernel: [  149.507119] ------------------------------------------------
kernel: [  149.507127] cat/3279 is leaving the kernel with locks still held!
kernel: [  149.507135] 1 lock held by cat/3279:
kernel: [  149.507141]  #0:  (scan_mutex){+.+.+.}, at: [<c110707c>] kmemleak_open+0x4c/0x80

problem is here:
static int kmemleak_open(struct inode *inode, struct file *file)
{
	int ret = 0;

	if (!atomic_read(&kmemleak_enabled))
		return -EBUSY;

	ret = mutex_lock_interruptible(&scan_mutex);
	if (ret < 0)
		goto out;
	if (file->f_mode & FMODE_READ) {
		ret = seq_open(file, &kmemleak_seq_ops);
		if (ret < 0)
			goto scan_unlock;
	}
>>-	return ret;

scan_unlock:
	mutex_unlock(&scan_mutex);
out:
	return ret;
}

we should not return before mutex_unlock(&scan_mutex);

	Sergey