In addition to the NOCOPY flag, add a DUP flag that
tells the transport to kmemdup() the buffer and free
it after the command completes.

Currently this is only supported for a single buffer
in a given command, but that could be extended if it
should be needed.
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>