net: qrtr: Refactor packet allocation

Extract the allocation and filling in the control message header fields
to a separate function in order to reuse this in subsequent patches.

Cc: Courtney Cavin <ccavin@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>