6lowpan: introduce lowpan_push_hc_data function

This patch introduce the lowpan_push_hc_data function to set data in
the iphc buffer.

It's a common case to set data and increase the buffer pointer. This
helper function can be used many times in header_compress function to
generate the iphc header.
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/ieee802154/6lowpan.h

@@ -298,6 +298,13 @@ static inline bool lowpan_fetch_skb(struct sk_buff *skb,
 	return false;
 }
 
+static inline void lowpan_push_hc_data(u8 **hc_ptr, const void *data,
+				       const size_t len)
+{
+	memcpy(*hc_ptr, data, len);
+	*hc_ptr += len;
+}
+
 typedef int (*skb_delivery_cb)(struct sk_buff *skb, struct net_device *dev);
 
 int lowpan_process_data(struct sk_buff *skb, struct net_device *dev,