greybus: embed message buffer into message structure

Embed the buffer for message data into the message structure itself.
This allows us to use a single allocation for each message, and
more importantly will allow us to derive the message structure
describing a message from the buffer itself.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>