LU-12232 test: call dt_sync after dd

LU-12232 test: call dt_sync after dd

In test_6 of replay-ost-single, calling wait_mds_ost_sync
after "dd" to sync the write transactions.

Test-Parameters: trivial alwaysuploadlogs \
mdtfilesystemtype=zfs ostfilesystemtype=zfs \
testlist=replay-ost-single,replay-ost-single,replay-ost-single

Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Change-Id: I26f6b1fe903c8b7b3f4e5fb698baffc4c3b85130
Reviewed-on: https://review.whamcloud.com/36772
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>