LU-924 test: sync client data before reboot server

LU-924 test: sync client data before reboot server

Lustre doesn't write client data synchronously (to avoid flooding sync
writes when there are many clients connecting, see exp_need_sync), so
if the server reboots before client data reachs disk, the client data
will be lost and client will be evicted after recovery, which is not
what we expected.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I21bcafe1f2630285c9108c0528467eb177e3449b
Reviewed-on: http://review.whamcloud.com/1888
Tested-by: Hudson
Reviewed-by: Chris Gearing <chris@whamcloud.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>