tcm_vhost: Convert I/O path to use target_submit_cmd_map_sgls

This patch converts tcm_vhost to use target_submit_cmd_map_sgls() for
I/O submission and mapping of pre-allocated SGL memory from incoming
virtio-scsi SGL memory -> se_cmd descriptors.

This includes removing the original open-coded fabric uses of target
core callers to support transport_generic_map_mem_to_cmd() between
target_setup_cmd_from_cdb() and transport_handle_cdb_direct() logic.

It also includes adding a handful of new tcm_vhost_cmnd member +
assignments in vhost_scsi_allocate_cmd() used from cmwq process
context I/O submission within tcm_vhost_submission_work()

(v2: Use renamed target_submit_cmd_map_sgls)

Reported-by: Christoph Hellwig <hch@lst.de>
Cc: Christoph Hellwig <hch@lst.de>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/vhost/tcm_vhost.h

@@ -5,6 +5,12 @@
 struct tcm_vhost_cmd {
 	/* Descriptor from vhost_get_vq_desc() for virt_queue segment */
 	int tvc_vq_desc;
+	/* virtio-scsi initiator task attribute */
+	int tvc_task_attr;
+	/* virtio-scsi initiator data direction */
+	enum dma_data_direction tvc_data_direction;
+	/* Expected data transfer length from virtio-scsi header */
+	u32 tvc_exp_data_len;
 	/* The Tag from include/linux/virtio_scsi.h:struct virtio_scsi_cmd_req */
 	u64 tvc_tag;
 	/* The number of scatterlists associated with this cmd */
@@ -17,6 +23,8 @@ struct tcm_vhost_cmd {
 	struct virtio_scsi_cmd_resp __user *tvc_resp;
 	/* Pointer to vhost_scsi for our device */
 	struct vhost_scsi *tvc_vhost;
+	/* Pointer to vhost nexus memory */
+	struct tcm_vhost_nexus *tvc_nexus;
 	/* The TCM I/O descriptor that is accessed via container_of() */
 	struct se_cmd tvc_se_cmd;
 	/* work item used for cmwq dispatch to tcm_vhost_submission_work() */