greybus: add response buffer to an operation

greybus: add response buffer to an operation

We need to track both request messages and response messages in
operations.  So add another gbuf (and payload pointer) field to
the operation structure, and rename them to indicate which one
is which.  Allow the creator specify the size of the response
buffer; just leave it a null pointer if the size is 0.

Define a new helper function gb_operation_gbuf_create() to
encapsulate creating either a request or a response buffer.

Any buffer associated with a connection will (eventually) have been
created as part of an operation.  So stash the operation pointer in
the gbuf as the context pointer.  Whether a buffer is for the
request or the response can be determined by pointer comparison.

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