mkfs: print zone count per device in summary

The zone size is printed in the list, zone counts are printed when the zones are reset but it would be nice to have them also in the final list. Something like:

```
...
Resetting device zones /dev/nullb0 (1000 zones) ...
...
Zoned device:       yes
  Zone size:        256.00MiB
...
Number of devices:  1
Devices:
   ID        SIZE  ZONES  PATH
    1   250.00GiB   1000  /dev/nullb0
```