Do not report bytes skipped by scan as issued.

Scan process may skip blocks based on their birth time, DVA, etc.
Traditionally those blocks were accounted as issued, that caused
reporting of hugely over-inflated numbers, having nothing to do
with actual disk I/O.  This change utilizes never used field in
struct dsl_scan_phys to account such skipped bytes, allowing to
report how much data were actually scrubbed/resilvered and what
is the actual I/O speed.  While formally it is an on-disk format
change, it should be compatible both ways, so should not need a
feature flag.

This should partially address the same issue as c85ac731a0, but
from a different perspective, complementing it.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Akash B <akash-b@hpe.com>
Signed-off-by:  Alexander Motin <mav@FreeBSD.org>
Sponsored by:   iXsystems, Inc.
Closes #15007

man/man8/zpool-scrub.8

@@ -26,7 +26,7 @@
 .\" Copyright 2017 Nexenta Systems, Inc.
 .\" Copyright (c) 2017 Open-E, Inc. All Rights Reserved.
 .\"
-.Dd July 25, 2021
+.Dd June 22, 2023
 .Dt ZPOOL-SCRUB 8
 .Os
 .
@@ -123,7 +123,7 @@ Status of pool with ongoing scrub:
 .No # Nm zpool Cm status
   ...
   scan: scrub in progress since Sun Jul 25 16:07:49 2021
-        403M scanned at 100M/s, 68.4M issued at 10.0M/s, 405M total
+        403M / 405M scanned at 100M/s, 68.4M / 405M issued at 10.0M/s
         0B repaired, 16.91% done, 00:00:04 to go
   ...
 .Ed