svcrdma: Update the synopsis of svc_rdma_read_data_item()

Since the RDMA Read I/O state is now contained in the recv_ctxt,
svc_rdma_build_read_data_item() can use that recv_ctxt to derive
that information rather than the other way around. This removes
another usage of the ri_readctxt field, enabling its removal in a
subsequent patch.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>