svcrdma: Update synopsis of svc_rdma_build_read_segment()

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

At the same time, the use of ri_rqst can similarly be replaced with
a passed-in function parameter.

Start with build_read_segment() because it is a common utility
function at the bottom of the Read chunk path.

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