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>
diff --git a/net/rxrpc/ar-key.c b/net/rxrpc/ar-key.c
index da3cc09..3f65716 100644
--- a/net/rxrpc/ar-key.c
+++ b/net/rxrpc/ar-key.c
@@ -896,15 +896,9 @@
 	if (optlen <= 0 || optlen > PAGE_SIZE - 1)
 		return -EINVAL;
 
-	description = kmalloc(optlen + 1, GFP_KERNEL);
-	if (!description)
-		return -ENOMEM;
-
-	if (copy_from_user(description, optval, optlen)) {
-		kfree(description);
-		return -EFAULT;
-	}
-	description[optlen] = 0;
+	description = memdup_user_nul(optval, optlen);
+	if (IS_ERR(description))
+		return PTR_ERR(description);
 
 	key = request_key(&key_type_rxrpc, description, NULL);
 	if (IS_ERR(key)) {
@@ -933,15 +927,9 @@
 	if (optlen <= 0 || optlen > PAGE_SIZE - 1)
 		return -EINVAL;
 
-	description = kmalloc(optlen + 1, GFP_KERNEL);
-	if (!description)
-		return -ENOMEM;
-
-	if (copy_from_user(description, optval, optlen)) {
-		kfree(description);
-		return -EFAULT;
-	}
-	description[optlen] = 0;
+	description = memdup_user_nul(optval, optlen);
+	if (IS_ERR(description))
+		return PTR_ERR(description);
 
 	key = request_key(&key_type_keyring, description, NULL);
 	if (IS_ERR(key)) {