LU-12232 test: commit before df

LU-12232 test: commit before df

In sub_test6 of replay_ost_single, the transactions at OSTs should
be committed to cleanup the test environment.

Test-Parameters: trivial
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Change-Id: Icbb06789855ab02252b7f1b0b9aff6bbb0f5f2e1
Reviewed-on: https://review.whamcloud.com/34808
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>