ddt: block scan until log is flushed, and flush aggressively

The dedup log does not have a stable cursor, so its not possible to
persist our current scan location within it across pool reloads.
Beccause of this, when walking (scanning), we can't treat it like just
another source of dedup entries.

Instead, when a scan is wanted, we switch to an aggressive flushing
mode, pushing out entries older than the scan start txg as fast as we
can, before starting the scan proper.

Entries after the scan start txg will be handled via other methods; the
DDT ZAPs and logs will be written as normal, and blocks not seen yet
will be offered to the scan machinery as normal.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: iXsystems, Inc.
Closes #15895

include/sys/ddt.h

@@ -294,6 +294,8 @@ typedef struct {
 	int32_t		ddt_log_flush_rate;	/* rolling log flush rate */
 	int32_t		ddt_log_flush_time_rate; /* avg time spent flushing */
 
+	uint64_t	ddt_flush_force_txg;	/* flush hard before this txg */
+
 	enum zio_checksum ddt_checksum;	/* checksum algorithm in use */
 	spa_t		*ddt_spa;	/* pool this ddt is on */
 	objset_t	*ddt_os;	/* ddt objset (always MOS) */
@@ -393,6 +395,9 @@ extern void ddt_create(spa_t *spa);
 extern int ddt_load(spa_t *spa);
 extern void ddt_unload(spa_t *spa);
 extern void ddt_sync(spa_t *spa, uint64_t txg);
+
+extern void ddt_walk_init(spa_t *spa, uint64_t txg);
+extern boolean_t ddt_walk_ready(spa_t *spa);
 extern int ddt_walk(spa_t *spa, ddt_bookmark_t *ddb,
     ddt_lightweight_entry_t *ddlwe);