LU-7428 tests: write superblock in conf-sanity test_84

LU-7428 tests: write superblock in conf-sanity test_84

In conf-sanity.sh test_84() a newly-formatted MDS filesystem's block
device is set read-only immediately after mount via replay_barrier(),
which may result in initial formatting or configs to be discarded,
resulting in a wide variety of different failure modes for this test.

Ensure that the superblock and other configuration logs are flushed
to disk before replay_barrier() is called, so that the MDS can mount
properly again later in the test.

Test-Parameters: testlist=conf-sanity,conf-sanity,conf-sanity,conf-sanity
Test-Parameters: testlist=conf-sanity,conf-sanity,conf-sanity,conf-sanity
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I95894b600ed7596c2014cba4f35fef4b443ebbe5
Reviewed-on: http://review.whamcloud.com/17371
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>