LU-3326 tests: sync after file deleted in run_*.sh

LU-3326 tests: sync after file deleted in run_*.sh

In run_*.sh, call sync to flush any dirty pages to OST
if MDT has not sent the object destroy request to it,
which will shrink the available disk space gotten before
running the tests and cause -EIO error.

This patch is back-ported from the following one:
Lustre-commit: 58c2a357202ba83d26ba9d8f1d9c093e3272e9a0
Lustre-change: http://review.whamcloud.com/8071

Test-Parameters: alwaysuploadlogs \
envdefinitions=SLOW=yes,ENABLE_QUOTA=yes,REQFAIL=100,DURATION=28800 \
clientdistro=el6 ossdistro=el6 mdsdistro=el6 clientarch=x86_64 \
ossarch=x86_64 mdsarch=x86_64 clientcount=4 osscount=2 mdscount=2 \
mdssizegb=10 ostsizegb=10 austeroptions=-R failover=true iscsi=1 \
testlist=recovery-mds-scale

Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Change-Id: I922774dcbf5546a9f5e3fe345bac954fd73d50a7
Signed-off-by: Jian Yu <jian.yu@intel.com>
Reviewed-on: http://review.whamcloud.com/11425
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>