svcrdma: Move write_info for Reply chunks into struct svc_rdma_send_ctxt

Since the RPC transaction's svc_rdma_send_ctxt will stay around for
the duration of the RDMA Write operation, the write_info structure
for the Reply chunk can reside in the request's svc_rdma_send_ctxt
instead of being allocated separately.

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