net/atm: fix proc_mpc_write 1 byte less calculated

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

Signed-off-by: Xiaobo Liu <cppcoffee@gmail.com>

net/atm/mpoa_proc.c

@@ -224,8 +224,11 @@ static ssize_t proc_mpc_write(struct file *file, const char __user *buff,
 			free_page((unsigned long)page);
 			return -EFAULT;
 		}
-		if (*p == '\0' || *p == '\n')
+
+		if (*p == '\0' || *p == '\n') {
+			len += 1
 			break;
+		}
 	}
 
 	*p = '\0';