kdbus: always pack message header as as single PAYLOAD_VEC item
According to PORTING-DBUS1 document [1], the message header in its
entirety must be contained in a single PAYLOAD_VEC item. What's more,
in case of memfd transport, message footer (which contains body signature
and offsets size) has to be attached at the end of message as a yet another
PAYLOAD_VEC item.
[1] https://cgit.freedesktop.org/systemd/systemd/tree/src/libsystemd/sd-bus/PORTING-DBUS1
Change-Id: I282589c0641c1eb97f874fcfd6e3bee6ecacc8ae