IB/rdmavt: add modify queue pair driver helpers

Low level drivers need to be able to check incoming attributes as well as be
able to adjust their private data on queue pair modification.  Add 2 driver
callbacks, check_modify_qp and modify_qp, to facilitate this.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>