IB/mad: Add const qualifiers to query only functions

The following functions only need read access to the data passed to them.

ib_mad_kernel_rmpp_agent
is_rmpp_data_mad
rcv_has_same_gid
ib_find_send_mad

Clarify with const specifiers

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