IB/hfi1: Refactor assign_ctxt() IOCTL

The IOCTL is a bit unwieldy.  Refactor to a common pattern.
Refactor the assign_ctxt() IOCTL.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>