zdb: don't print log spacemap stats in pools without the feature

Creating a pool with not features enabled and running
`zdb -mmmmmm on` it before the patch:

```
Log Space Maps in Pool:

Log Space Map Obsolete Entry Statistics:
0        valid entries out of 0        - txg 0
0        valid entries out of 0        - total
```

After this patch the above output goes away.

Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Sara Hartse <sara.hartse@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #9048

cmd/zdb/zdb.c

@@ -1204,6 +1204,9 @@ dump_metaslabs(spa_t *spa)
 static void
 dump_log_spacemaps(spa_t *spa)
 {
+	if (!spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP))
+		return;
+
 	(void) printf("\nLog Space Maps in Pool:\n");
 	for (spa_log_sm_t *sls = avl_first(&spa->spa_sm_logs_by_txg);
 	    sls; sls = AVL_NEXT(&spa->spa_sm_logs_by_txg, sls)) {
@@ -5823,6 +5826,9 @@ log_spacemap_obsolete_stats_cb(spa_t *spa, space_map_entry_t *sme,
 static void
 dump_log_spacemap_obsolete_stats(spa_t *spa)
 {
+	if (!spa_feature_is_active(spa, SPA_FEATURE_LOG_SPACEMAP))
+		return;
+
 	log_sm_obsolete_stats_arg_t lsos;
 	bzero(&lsos, sizeof (lsos));