rdma_cm: add rdma_consumer_reject_data helper function

rdma_consumer_reject_data() will return the private data pointer
and length if any is available.

Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>