net: qrtr: Clean up control packet handling

As the message header generation is deferred the internal functions for
generating control packets can be simplified.

This patch modifies qrtr_alloc_ctrl_packet() to, in addition to the
sk_buff, return a reference to a struct qrtr_ctrl_pkt, which clarifies
and simplifies the helpers to the point that these functions can be
folded back into the callers.

Change-Id: I1681c75b3e2015b9b7d1cb6b4819e52d96299573
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Patch-mainline: netdev @ 06/09/17, 23:03
Signed-off-by: Chris Lew <clew@codeaurora.org>