LU-3021 ldiskfs: to not verify preallocation in umount path

LU-3021 ldiskfs: to not verify preallocation in umount path

When recovery test is running, device rdonly flag is set and all
updates will be erred out so on-disk data can be inconsistent with
memory data.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: I69d3fa462c69f5f85978265075165b488661e863
Reviewed-on: http://review.whamcloud.com/5883
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>