Fix and disable blocks statistics during scrub

Block statistics calculation during scrub I/O issue in case of sorted
scrub accounted ditto blocks several times. Embedded blocks on other
side were not accounted at all. This change moves the accounting from
issue to scan stage, that fixes both problems and also allows to avoid
pool-wide locking and the lock contention it created.

Since this statistics is quite specific and is not even exposed now
anywhere, disable its calculation by default to not waste CPU time.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #13579