Add ability to scrub from last scrubbed txg

Some users might want to scrub only new data because they would like
to know if the new write wasn't corrupted.  This PR adds possibility
scrub only newly written data.

This introduces new `last_scrubbed_txg` property, indicating the
transaction group (TXG) up to which the most recent scrub operation
has checked and repaired the dataset, so users can run scrub only
from the last saved point. We use a scn_max_txg and scn_min_txg
which are already built into scrub, to accomplish that.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mariusz Zaborski <mariusz.zaborski@klarasystems.com>
Sponsored-By: Wasabi Technology, Inc.
Sponsored-By: Klara Inc.
Closes #16301

include/sys/spa_impl.h

@@ -318,6 +318,7 @@ struct spa {
 	uint64_t	spa_scan_pass_scrub_spent_paused; /* total paused */
 	uint64_t	spa_scan_pass_exam;	/* examined bytes per pass */
 	uint64_t	spa_scan_pass_issued;	/* issued bytes per pass */
+	uint64_t	spa_scrubbed_last_txg;	/* last txg scrubbed */
 
 	/* error scrub pause time in milliseconds */
 	uint64_t	spa_scan_pass_errorscrub_pause;