IB/mad: Change validate_mad signature arguments

validate_mad only needs read access to the MAD header, not write access
to the entire mad struct, so replace struct ib_mad with const struct
ib_mad_hdr

Reviewed-By: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Reviewed-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>