IB/mad: Create a generic helper for DR SMP Recv processing

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 DR SMP Recv messages which
can be used by both IB and OPA SMP code.

Use this function in the current IB function smi_handle_dr_smp_recv.

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