DDT: Skip DDT log lookups in ddt_prune_walk()

DDT log is not explicitly locked, and serialized only by the TXG
sync process.  Since that stage of ddt_prune_walk() is not in sync
with the TXG sync process, we can't access the DDT log there.  We
could possibly lock it, but it seems we can just skip it, since it
is only a performance optimization, while the final LOG lookup is
be done by prune_candidates_sync() any way.

Reviewed-by: Rob Norris <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Alexander Motin <[email protected]>
Closes #18889

module/zfs/ddt.c

@@ -2860,13 +2860,6 @@ ddt_prune_walk(spa_t *spa, uint64_t cutoff, ddt_age_histo_t *histogram)
 		uint64_t class_start =
 		    ddlwe.ddlwe_phys.ddp_flat.ddp_class_start;
 
-		/*
-		 * If this entry is on the log, then the stored entry is stale
-		 * and we should skip it.
-		 */
-		if (ddt_log_find_key(ddt, &ddlwe.ddlwe_key, NULL, NULL))
-			continue;
-
 		/* prune older entries */
 		if (pruning && class_start < cutoff) {
 			if (candidates++ >= zfs_ddt_prunes_per_txg) {