some NetBufferUtility revamp.

- introduced base NetBufferFieldReader which deals with obtaining a continuguous field to deal with.
 - renamed Detach() to Sync() to better express the fact that we may be writing to the buffer.
 - Fixed IPv4's and ICMP's checksum calculation as it assumed the underlying header was contiguous.
 - ICMP is now able to reply to ICMP Echo Requests which were split across data nodes.
 - in split_buffer(), make sure we were able to trim() the new buffer before damaging the original one.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@20657 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/add-ons/kernel/network/stack/net_buffer.cpp

@@ -464,9 +464,10 @@ split_buffer(net_buffer *from, uint32 offset)
 
 	TRACE(("split_buffer(buffer %p -> %p, offset %ld)\n", from, buffer, offset));
 
-	if (remove_header(from, offset) == B_OK
-		&& trim_data(buffer, offset) == B_OK)
-		return buffer;
+	if (trim_data(buffer, offset) == B_OK) {
+		if (remove_header(from, offset) == B_OK)
+			return buffer;
+	}
 
 	free_buffer(buffer);
 	return NULL;