IB/mad: Create a generic helper for DR forwarding checks

IB and OPA SMPs share the same processing algorithm but have different header
formats and permissive LID detection.

Add a helper function which is generic to processing the DR forwarding checks which
can be used by both IB and OPA SMP code.

Use this function in the current IB function smi_check_forward_dr_smp.

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