selftests/net: revert the zerocopy Rx path for PF_RDS

In preparation for optimized reception of zerocopy completion,
revert the Rx side changes introduced by Commit dfb8434b
("selftests/net: add zerocopy support for PF_RDS test case")

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>