ddt: Comment out some unused variables

This is not associated with a specific upstream commit but apparently
a local diff applied as part of:

commit 7657eaaeb514e54249cd62a3cfe420e0c94693f9
Merge: b4d938024c78 e2df9bb44109
Author: Martin Matuska <[email protected]>
Date:   Mon Sep 9 18:13:02 2024 +0200

    zfs: merge openzfs/zfs@b10992582

    Notable upstream pull request merges:
     openzfs#15892 -multiple Fast Dedup: Introduce the FDT on-disk format and feature flag
     openzfs#15893 -multiple Fast Dedup: “flat” DDT entry format
     openzfs#15895 -multiple Fast Dedup: FDT-log feature
     openzfs#16239 6be8bf5 zpool: Provide GUID to zpool-reguid(8) with -g
     openzfs#16277 -multiple Fast Dedup: prune unique entries
     openzfs#16316 5807de9 Fix null ptr deref when renaming a zvol with snaps and snapdev=visible
     openzfs#16343 77a797a Enable L2 cache of all (MRU+MFU) metadata but MFU data only
     openzfs#16446 83f3592 FreeBSD: fix build without kernel option MAC
     openzfs#16449 963e6c9 Fix incorrect error report on vdev attach/replace
     openzfs#16505 b109925 spa_prop_get: require caller to supply output nvlist

    Obtained from:  OpenZFS
    OpenZFS commit: b109925

module/zfs/ddt.c

@@ -2609,7 +2609,11 @@ ddt_prune_walk(spa_t *spa, uint64_t cutoff, ddt_age_histo_t *histogram)
 	};
 	ddt_lightweight_entry_t ddlwe = {0};
 	int error;
-	int total = 0, valid = 0;
+	/* 
+	 * TBD: Unused variable total 
+	 * int total = 0, valid = 0;
+	 */
+	int valid = 0;
 	int candidates = 0;
 	uint64_t now = gethrestime_sec();
 	ddt_prune_info_t dpi;
@@ -2633,7 +2637,10 @@ ddt_prune_walk(spa_t *spa, uint64_t cutoff, ddt_age_histo_t *histogram)
 
 		if (spa_shutting_down(spa) || issig())
 			break;
-		total++;
+		/*
+		 * TBD: Unused variable total
+		 * total++;
+		 */
 
 		ASSERT(ddt->ddt_flags & DDT_FLAG_FLAT);
 		ASSERT3U(ddlwe.ddlwe_phys.ddp_flat.ddp_refcnt, <=, 1);