Add 'zero-copy' in libiscsi for reads.

It is not real zero-copy since the data is still copied in the kernel,
but it avoids copying the data inside libiscsi as well as in the callback.

For SCSI tasks that will return data from the target, the application can now
specify application buffers for libiscsi to read the data directly into.
This is done by calling scsi_task_add_data_in_buffer(task, ...

These buffers need not be linear, you can specify different areas to read into
by calling this function several times.

See examples/iscsiclient.c for an example.

lib/scsi-lowlevel.c

@@ -936,3 +936,50 @@ scsi_get_task_private_ptr(struct scsi_task *task)
 {
 	return task->ptr;
 }
+
+
+
+struct scsi_data_buffer {
+       struct scsi_data_buffer *next;
+       int len;
+       unsigned char *data;
+};
+
+int
+scsi_task_add_data_in_buffer(struct scsi_task *task, int len, unsigned char *buf)
+{
+	struct scsi_data_buffer *data_buf;
+
+	data_buf = scsi_malloc(task, sizeof(struct scsi_data_buffer));
+	if (data_buf == NULL) {
+		return -1;
+	}
+
+	data_buf->len  = len;
+	data_buf->data = buf;
+
+	SLIST_ADD_END(&task->in_buffers, data_buf);
+	return 0;
+}
+
+unsigned char *
+scsi_task_get_data_in_buffer(struct scsi_task *task, uint32_t pos, ssize_t *count)
+{
+	struct scsi_data_buffer *sdb;
+
+	sdb = task->in_buffers;
+	if (sdb == NULL) {
+		return NULL;
+	}
+
+	while (pos >= sdb->len) {
+	      pos -= sdb->len;
+	      sdb  = sdb->next;
+	}
+
+	if (count && *count > sdb->len - pos) {
+		*count = sdb->len - pos;
+	}
+
+	return &sdb->data[pos];
+}