NTB: ntb_tool: Switch to memdup_user_nul() helper

Use memdup_user_nul() helper instead of open-coding to simplify the code.
Signed-off-by: Ruan Jinjie <ruanjinjie@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>

drivers/ntb/test/ntb_tool.c

@@ -370,16 +370,9 @@ static ssize_t tool_fn_write(struct tool_ctx *tc,
 	if (*offp)
 		return 0;
 
-	buf = kmalloc(size + 1, GFP_KERNEL);
-	if (!buf)
-		return -ENOMEM;
-
-	if (copy_from_user(buf, ubuf, size)) {
-		kfree(buf);
-		return -EFAULT;
-	}
-
-	buf[size] = 0;
+	buf = memdup_user_nul(ubuf, size);
+	if (IS_ERR(buf))
+		return PTR_ERR(buf);
 
 	n = sscanf(buf, "%c %lli", &cmd, &bits);