IB/rdmavt: Move driver helper functions to a common structure

Drivers are going to need to provide multiple functions for rdmavt to
call in to. We already have one, so go ahead and push this into a
data structure designated for driver supplied functions.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>