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.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I17c0d440e4c89cae1780de63ca2b3a0d69de298b
Reviewed-on: http://review.whamcloud.com/7591
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alexander Boyko <alexander_boyko@xyratex.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>