iscsi-target: Convert transport drivers to signal rdma_shutdown

Instead of special casing the handful of callers that check for
iser-target rdma verbs specific shutdown, use a simple flag at
iscsit_transport->rdma_shutdown so each driver can signal this.

Also, update iscsi-target/tcp + cxgbit to rdma_shutdown = false.

Cc: Varun Prakash <varun@chelsio.com>
Cc: Hariprasad Shenai <hariprasad@chelsio.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/cxgbit/cxgbit_main.c

@@ -625,6 +625,7 @@ static enum target_prot_op cxgbit_get_sup_prot_ops(struct iscsi_conn *conn)
 static struct iscsit_transport cxgbit_transport = {
 	.name			= DRV_NAME,
 	.transport_type		= ISCSI_HW_OFFLOAD,
+	.rdma_shutdown		= false,
 	.priv_size		= sizeof(struct cxgbit_cmd),
 	.owner			= THIS_MODULE,
 	.iscsit_setup_np	= cxgbit_setup_np,