net/atm: fix proc_mpc_write incorrect return value

[ Upstream commit d8bde3b ]

Then the input contains '\0' or '\n', proc_mpc_write has read them,
so the return value needs +1.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Xiaobo Liu <cppcoffee@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>

net/atm/mpoa_proc.c

@@ -219,11 +219,12 @@ static ssize_t proc_mpc_write(struct file *file, const char __user *buff,
 	if (!page)
 		return -ENOMEM;
 
-	for (p = page, len = 0; len < nbytes; p++, len++) {
+	for (p = page, len = 0; len < nbytes; p++) {
 		if (get_user(*p, buff++)) {
 			free_page((unsigned long)page);
 			return -EFAULT;
 		}
+		len += 1;
 		if (*p == '\0' || *p == '\n')
 			break;
 	}