How to mark packets that were sampled when tracing is enabled

[sflow]

Pim pointed out that when the sflow worker traces a packet it should probably indicate if that packet was sampled and if the sample was successfully enqueued or not. Seems like a good idea, but I'm not sure what bits we are allowed to set in the packet-buffer so I just added a TODO comment for now:

vpp-sflow/sflow/node.c

Lines 123 to 126 in d4f7472

	// TODO: what bit in the buffer can we set right here to indicate
	// that this packet was sampled (and perhaps another bit to say if it
	// was dropped or sucessfully enqueued)? That way we can check it
	// below if the packet is traced, and indicate that in the trace output.