IB/core cleanup: Add const to args - agent_send_response

In order to support constant callers of agent_send_response we add const
specifiers to the its pointer arguments.

Adjust the call tree accordingly.

Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Hal Rosenstock <hal@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>