ZTS: Fix vdev_zaps_005_pos on CentOS 6

[behlendorf]

Motivation and Context

Resolve vdev_zaps_005_pos test failure when running on CentOS 6.
Observed by the CI.

Description

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.

How Has This Been Tested?

Locally in a CentOS 6 VM.

$ ./scripts/zfs-tests.sh -t tests/functional/vdev_zaps/vdev_zaps_005_pos
Test: /home/behlendo/src/git/zfs/tests/zfs-tests/tests/functional/vdev_zaps/setup (run as root) [00:00] [PASS]
Test: /home/behlendo/src/git/zfs/tests/zfs-tests/tests/functional/vdev_zaps/vdev_zaps_005_pos (run as root) [00:07] [PASS]
Test: /home/behlendo/src/git/zfs/tests/zfs-tests/tests/functional/vdev_zaps/cleanup (run as root) [00:00] [PASS]

Results Summary
PASS	   3

Running Time:	00:00:08
Percent passed:	100.0%
Log directory:	/var/tmp/test_results/20190821T141509

Tests with results other than PASS that are expected:

Tests with result of PASS that are unexpected:

Tests with results other than PASS that are unexpected:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov]

Codecov Report

Merging #9199 into master will decrease coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #9199      +/-   ##
==========================================
- Coverage   79.22%   78.98%   -0.24%     
==========================================
  Files         400      400              
  Lines      122001   122001              
==========================================
- Hits        96656    96368     -288     
- Misses      25345    25633     +288

Flag	Coverage Δ
#kernel	79.74% <ø> (-0.01%)	⬇️
#user	66.57% <ø> (-0.47%)	⬇️

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