Teach zdb about DMU_OT_ERROR_LOG objects

With the persistent error log feature we need to account for
spa_errlog_{scrub, last} containing mappings to other error log objects,
which need to be marked as in-use as well.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes openzfs#14442 
Closes openzfs#14434

cmd/zdb/zdb.c

@@ -7509,6 +7509,19 @@ mos_leak_log_spacemaps(spa_t *spa)
 		mos_obj_refd(sls->sls_sm_obj);
 }
 
+static void
+errorlog_count_refd(objset_t *mos, uint64_t errlog)
+{
+	zap_cursor_t zc;
+	zap_attribute_t za;
+	for (zap_cursor_init(&zc, mos, errlog);
+	    zap_cursor_retrieve(&zc, &za) == 0;
+	    zap_cursor_advance(&zc)) {
+		mos_obj_refd(za.za_first_integer);
+	}
+	zap_cursor_fini(&zc);
+}
+
 static int
 dump_mos_leaks(spa_t *spa)
 {
@@ -7529,6 +7542,12 @@ dump_mos_leaks(spa_t *spa)
 	mos_obj_refd(spa->spa_history);
 	mos_obj_refd(spa->spa_errlog_last);
 	mos_obj_refd(spa->spa_errlog_scrub);
+
+	if (!spa_feature_is_enabled(spa, SPA_FEATURE_HEAD_ERRLOG)) {
+		errorlog_count_refd(mos, spa->spa_errlog_last);
+		errorlog_count_refd(mos, spa->spa_errlog_scrub);
+	}
+
 	mos_obj_refd(spa->spa_all_vdev_zaps);
 	mos_obj_refd(spa->spa_dsl_pool->dp_bptree_obj);
 	mos_obj_refd(spa->spa_dsl_pool->dp_tmp_userrefs_obj);