scsi: fix the {host,target,device}_blocked counter mess

Seems like these counters are missing any sort of synchronization for
updates, as a over 10 year old comment from me noted.  Fix this by
using atomic counters, and while we're at it also make sure they are
in the same cacheline as the _busy counters and not needlessly stored
to in every I/O completion.

With the new model the _busy counters can temporarily go negative,
so all the readers are updated to check for > 0 values.  Longer
term every successful I/O completion will reset the counters to zero,
so the temporarily negative values will not cause any harm.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Webb Scales <webbnh@hp.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Tested-by: Bart Van Assche <bvanassche@acm.org>
Tested-by: Robert Elliott <elliott@hp.com>

include/scsi/scsi_host.h

@@ -583,6 +583,8 @@ struct Scsi_Host {
 	struct blk_queue_tag	*bqt;
 
 	atomic_t host_busy;		   /* commands actually active on low-level */
+	atomic_t host_blocked;
+
 	unsigned int host_failed;	   /* commands that failed.
 					      protected by host_lock */
 	unsigned int host_eh_scheduled;    /* EH scheduled without command */
@@ -681,11 +683,6 @@ struct Scsi_Host {
 	 */
 	struct workqueue_struct *tmf_work_q;
 
-	/*
-	 * Host has rejected a command because it was busy.
-	 */
-	unsigned int host_blocked;
-
 	/*
 	 * Value host_blocked counts down from
 	 */