ddt: lookup and log stats

Adds per-DDT stats counting lookups and where they were serviced from
(either log or backing zap), number of log entries in memory, and flow
rates.

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

include/sys/ddt.h

@@ -296,6 +296,8 @@ typedef struct {
 
 	uint64_t	ddt_flush_force_txg;	/* flush hard before this txg */
 
+	kstat_t		*ddt_ksp;	/* kstats context */
+
 	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) */