iser-target: Kill struct isert_rdma_wr

There is exactly one instance per struct isert_cmd, so merge the two to
simplify everyones life.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>