svcrdma: Clean up Read chunk path

Simplify the error handling at the tail of recv_read_chunk() by
re-arranging rq_pages[] housekeeping and documenting it properly.

NB: In this path, svc_rdma_recvfrom returns zero. Therefore no
subsequent reply processing is done on the svc_rqstp, and thus the
rq_respages field does not need to be updated.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>