scsi: read host_busy via scsi_host_busy()

No functional change.

Just introduce scsi_host_busy() and replace the direct read of
scsi_host->host_busy with this new API.

Cc: Omar Sandoval <osandov@fb.com>,
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>,
Cc: James Bottomley <james.bottomley@hansenpartnership.com>,
Cc: Christoph Hellwig <hch@lst.de>,
Cc: Don Brace <don.brace@microsemi.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Laurence Oberman <loberman@redhat.com>
Cc: Bart Van Assche <bart.vanassche@wdc.com>

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
diff --git a/drivers/scsi/scsi.c b/drivers/scsi/scsi.c
index 70ef3c3..fc1356d 100644
--- a/drivers/scsi/scsi.c
+++ b/drivers/scsi/scsi.c
@@ -167,7 +167,7 @@ void scsi_log_completion(struct scsi_cmnd *cmd, int disposition)
 			if (level > 3)
 				scmd_printk(KERN_INFO, cmd,
 					    "scsi host busy %d failed %d\n",
-					    atomic_read(&cmd->device->host->host_busy),
+					    scsi_host_busy(cmd->device->host),
 					    cmd->device->host->host_failed);
 		}
 	}