Add DDT prune command

Requires the new 'flat' physical data which has the start
time for a class entry.

The amount to prune can be based on a target percentage of
the unique entries or based on the age (i.e., every entry
older than N days).

Sponsored-by: Klara, Inc.
Sponsored-by: iXsystems, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Closes #16277

module/zfs/zio.c

@@ -3859,6 +3859,16 @@ zio_ddt_free(zio_t *zio)
 	}
 	ddt_exit(ddt);
 
+	/*
+	 * When no entry was found, it must have been pruned,
+	 * so we can free it now instead of decrementing the
+	 * refcount in the DDT.
+	 */
+	if (!dde) {
+		BP_SET_DEDUP(bp, 0);
+		zio->io_pipeline |= ZIO_STAGE_DVA_FREE;
+	}
+
 	return (zio);
 }