LU-8981 test: sanity 311 check is too strict

LU-8981 test: sanity 311 check is too strict

System may be too busy to destroy unlinked objs in time, which
cause sanity fail, let's use a smaller value to not fail autotest.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: Ibbe543f279c3548176c53b3fdb7b8048ea08931f
Reviewed-on: https://review.whamcloud.com/24814
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>