benchmark: cache the kdbus msg and update only memfd item offset if needed

benchmark: cache the kdbus msg and update only memfd item offset if needed

No need to create the kdbus msg every time, and if we are sending a
memfd, then just store its item offset, patch it later with the right
memfd fd+data, seal it, then send it.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>