ZTS: Fix vdev_zaps_005_pos on CentOS 6

The ancient version of blkid (v2.17.2) used in CentOS 6 will not
detect the newly created pool unless it has been written to.
Force a pool sync so zpool import will detect the newly created
pool.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #9199