rdma core: Add rdma_rw_mr_payload()

The amount of payload per MR depends on device capabilities and
the memory registration mode in use. The new rdma_rw API hides both,
making it difficult for ULPs to determine how large their transport
send queues need to be.

Expose the MR payload information via a new API.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Acked-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>