IB/hfi1: Create inline to get extended headers

This paves the way for another patch that reacts to a
flush sdma completion for RC.

Fixes: 81cd3891 ("IB/hfi1: Add support for 16B Management Packets")
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>