IB/rdmavt: Add completion queue functions

Brings in completion queue functionality. A kthread worker is added to
the rvt_dev_info to serve as a worker for completion queues.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Harish Chegondi <harish.chegondi@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>