Wrap vdev_count_verify_zaps() with ZFS_DEBUG

Commit e0ab3ab introduced two blocks of code which are only needed
when debugging is enabled. These blocks should be wrapped with
ZFS_DEBUG for clarity and to prevent unused variable warnings in
a production build.

Signed-off-by: Don Brady <don.brady@intel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4515