net: qrtr: Use sk_buff->cb in receive path

Rather than parsing the header of incoming messages throughout the
implementation do it once when we retrieve the message and store the
relevant information in the "cb" member of the sk_buff.

This allows us to, in a later commit, decode version 2 messages into
this same structure.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>