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

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