svcrdma: Update synopsis of svc_rdma_build_read_chunk()

Since the RDMA Read I/O state is now contained in the recv_ctxt,
svc_rdma_build_read_chunk() 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>