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>

drivers/scsi/mpt3sas/mpt3sas_base.c

@@ -3250,7 +3250,7 @@ _base_recovery_check(struct MPT3SAS_ADAPTER *ioc)
 	 * See _wait_for_commands_to_complete() call with regards to this code.
 	 */
 	if (ioc->shost_recovery && ioc->pending_io_count) {
-		ioc->pending_io_count = atomic_read(&ioc->shost->host_busy);
+		ioc->pending_io_count = scsi_host_busy(ioc->shost);
 		if (ioc->pending_io_count == 0)
 			wake_up(&ioc->reset_wq);
 	}
@@ -6857,7 +6857,7 @@ mpt3sas_wait_for_commands_to_complete(struct MPT3SAS_ADAPTER *ioc)
 		return;
 
 	/* pending command count */
-	ioc->pending_io_count = atomic_read(&ioc->shost->host_busy);
+	ioc->pending_io_count = scsi_host_busy(ioc->shost);
 
 	if (!ioc->pending_io_count)
 		return;