crypto: hisilicon - switch to memdup_user_nul()

Use memdup_user_nul() helper instead of open-coding to
simplify the code.

v1-->v2:
   fixed patch title error
v2-->v3:
   return the actual error
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

drivers/crypto/hisilicon/qm.c

@@ -1603,16 +1603,9 @@ static ssize_t qm_cmd_write(struct file *filp, const char __user *buffer,
 	if (count > QM_DBG_WRITE_LEN)
 		return -ENOSPC;
 
-	cmd_buf = kzalloc(count + 1, GFP_KERNEL);
-	if (!cmd_buf)
-		return -ENOMEM;
-
-	if (copy_from_user(cmd_buf, buffer, count)) {
-		kfree(cmd_buf);
-		return -EFAULT;
-	}
-
-	cmd_buf[count] = '\0';
+	cmd_buf = memdup_user_nul(buffer, count);
+	if (IS_ERR(cmd_buf))
+		return PTR_ERR(cmd_buf);
 
 	cmd_buf_tmp = strchr(cmd_buf, '\n');
 	if (cmd_buf_tmp) {