svcrdma: Copy construction of svc_rqst::rq_arg to rdma_read_complete()

Once a set of RDMA Reads are complete, the Read completion handler
will poke the transport to trigger a second call to
svc_rdma_recvfrom(). recvfrom() will then merge the RDMA Read
payloads with the previously received RPC header to form a completed
RPC Call message.

The new code is copied from the svc_rdma_process_read_list() path.
A subsequent patch will make use of this code and remove the code
that this was copied from (svc_rdma_rw.c).

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