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

include/sys/dsl_scan.h

@@ -61,7 +61,7 @@ typedef struct dsl_scan_phys {
 	uint64_t scn_end_time;
 	uint64_t scn_to_examine; /* total bytes to be scanned */
 	uint64_t scn_examined; /* bytes scanned so far */
-	uint64_t scn_to_process;
+	uint64_t scn_skipped;	/* bytes skipped by scanner */
 	uint64_t scn_processed;
 	uint64_t scn_errors;	/* scan I/O error count */
 	uint64_t scn_ddt_class_max;

include/sys/fs/zfs.h

@@ -1088,7 +1088,7 @@ typedef struct pool_scan_stat {
 	uint64_t	pss_end_time;	/* scan end time */
 	uint64_t	pss_to_examine;	/* total bytes to scan */
 	uint64_t	pss_examined;	/* total bytes located by scanner */
-	uint64_t	pss_to_process; /* total bytes to process */
+	uint64_t	pss_skipped;	/* total bytes skipped by scanner */
 	uint64_t	pss_processed;	/* total processed bytes */
 	uint64_t	pss_errors;	/* scan errors	*/
 
@@ -1152,6 +1152,7 @@ typedef struct vdev_rebuild_stat {
 	uint64_t vrs_pass_time_ms;	/* pass run time (millisecs) */
 	uint64_t vrs_pass_bytes_scanned; /* bytes scanned since start/resume */
 	uint64_t vrs_pass_bytes_issued;	/* bytes rebuilt since start/resume */
+	uint64_t vrs_pass_bytes_skipped; /* bytes skipped since start/resume */
 } vdev_rebuild_stat_t;
 
 /*

include/sys/vdev_rebuild.h

@@ -79,6 +79,7 @@ typedef struct vdev_rebuild {
 	uint64_t	vr_pass_start_time;
 	uint64_t	vr_pass_bytes_scanned;
 	uint64_t	vr_pass_bytes_issued;
+	uint64_t	vr_pass_bytes_skipped;
 
 	/* On-disk state updated by vdev_rebuild_zap_update_sync() */
 	vdev_rebuild_phys_t vr_rebuild_phys;