RDMA/core: expose affinity mappings per completion vector

This will allow ULPs to intelligently locate threads based
on completion vector cpu affinity mappings. In case the
driver does not expose a get_vector_affinity callout, return
NULL so the caller can maintain a fallback logic.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Acked-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>