cam: Add comment about recovery ccbs

[imp]

Summary

SS_START and higher actions (currently only SS_TUR) allocate a recovery
CCB to send a command to the periph. Add a quick comment about that here.

Sponsored by: Netflix

sys/cam/scsi/scsi_all.h

 struct vmbus_softc {
         void                        (*vmbus_event_proc)(struct vmbus_softc *, int);
         u_long                  *vmbus_tx_evtflags;
                                                 /* event flags to host */
         void                    *vmbus_mnf2;    /* monitored by host */
 
         u_long                  *vmbus_rx_evtflags;
                                                 /* compat evtflgs from host */
+        struct hv_vmbus_channel **vmbus_chmap;
         struct vmbus_msghc_ctx  *vmbus_msg_hc;
         struct vmbus_pcpu_data  vmbus_pcpu[MAXCPU];
 
         /*
          * Rarely used fields
          */
 
         device_t                vmbus_dev;