IB/mad cleanup: Generalize processing of MAD data

ib_find_send_mad only needs access to the MAD header not the full IB MAD.
Change the local variable to ib_mad_hdr and change the corresponding cast.

This allows for clean usage of this function with both IB and OPA MADs because
OPA MADs carry the same header as IB MADs.

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