LU-3719 ldiskfs: adjust s_mb_prealloc_table_size correctly

LU-3719 ldiskfs: adjust s_mb_prealloc_table_size correctly

When mb prealloc table item value is not valid, the
s_mb_prealloc_table_size should be adjust accordingly.
Port patch to rhel7 & sles12.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alexander Boyko <alexander_boyko@xyratex.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I5aa3a32a6a3aedc70160409c6443746fd2ccbbc9
Reviewed-on: https://review.whamcloud.com/27748
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>