[SCSI] scsi_dh_alua: backoff alua rtpg retry linearly vs. geometrically

Currently the backoff algorithm for when to retry alua rtpg
requests progresses geometrically as so:

2, 4, 8, 16, 32, 64... seconds.

This progression can lead to un-needed delay in retrying
alua rtpg requests when the rtpgs are delayed.  A less
aggressive backoff algorithm that is additive would not
lead to such large jumps when delays start getting long, but
would backoff linearly:

2, 4, 6, 8, 10... seconds.

Signed-off-by: Martin George <marting@netapp.com>
Signed-off-by: Rob Evers <revers@redhat.com>
Reviewed-by: Babu Moger <babu.moger@netapp.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>

drivers/scsi/device_handler/scsi_dh_alua.c

@@ -527,7 +527,7 @@ static int alua_rtpg(struct scsi_device *sdev, struct alua_dh_data *h)
 	unsigned char *ucp;
 	unsigned err;
 	bool rtpg_ext_hdr_req = 1;
-	unsigned long expiry, interval = 1000;
+	unsigned long expiry, interval = 0;
 	unsigned int tpg_desc_tbl_off;
 	unsigned char orig_transition_tmo;
 
@@ -632,7 +632,7 @@ static int alua_rtpg(struct scsi_device *sdev, struct alua_dh_data *h)
 	case TPGS_STATE_TRANSITIONING:
 		if (time_before(jiffies, expiry)) {
 			/* State transition, retry */
-			interval *= 2;
+			interval += 2000;
 			msleep(interval);
 			goto retry;
 		}