Defer writing the message header to the skb until its time to enqueue
the packet. As the receive path is reworked to decode the message header
as it's received from the transport and only pass around the payload in
the skb this change means that we do not have to fill out the full
message header just to decode it immediately in qrtr_local_enqueue().

In the future this change also makes it possible to prepend message
headers based on the version of each link.
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>