convert a bunch of open-coded instances of memdup_user_nul()

A _lot_ of ->write() instances were open-coding it; some are
converted to memdup_user_nul(), a lot more remain...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/sbus/char/openprom.c

@@ -390,16 +390,9 @@ static int copyin_string(char __user *user, size_t len, char **ptr)
 	if ((ssize_t)len < 0 || (ssize_t)(len + 1) < 0)
 		return -EINVAL;
 
-	tmp = kmalloc(len + 1, GFP_KERNEL);
-	if (!tmp)
-		return -ENOMEM;
-
-	if (copy_from_user(tmp, user, len)) {
-		kfree(tmp);
-		return -EFAULT;
-	}
-
-	tmp[len] = '\0';
+	tmp = memdup_user_nul(user, len);
+	if (IS_ERR(tmp))
+		return PTR_ERR(tmp);
 
 	*ptr = tmp;