new helper: copy_to_iter_full()

... and convert copy_linear_skb() to using that.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/uio.h

@@ -205,6 +205,16 @@ size_t copy_from_iter(void *addr, size_t bytes, struct iov_iter *i)
 	return 0;
 }
 
+static __always_inline __must_check
+bool copy_to_iter_full(const void *addr, size_t bytes, struct iov_iter *i)
+{
+	size_t copied = copy_to_iter(addr, bytes, i);
+	if (likely(copied == bytes))
+		return true;
+	iov_iter_revert(i, copied);
+	return false;
+}
+
 static __always_inline __must_check
 bool copy_from_iter_full(void *addr, size_t bytes, struct iov_iter *i)
 {

include/net/udp.h

@@ -379,14 +379,7 @@ static inline bool udp_skb_is_linear(struct sk_buff *skb)
 static inline int copy_linear_skb(struct sk_buff *skb, int len, int off,
 				  struct iov_iter *to)
 {
-	int n;
-
-	n = copy_to_iter(skb->data + off, len, to);
-	if (n == len)
-		return 0;
-
-	iov_iter_revert(to, n);
-	return -EFAULT;
+	return copy_to_iter_full(skb->data + off, len, to) ? 0 : -EFAULT;
 }
 
 /*