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.

ported to SLES11 SP[23]

Lustre-change: http://review.whamcloud.com/7591
Lustre-commit: 6c9f72a1e537826be2b889feba1ff8c5b5db2e77

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Change-Id: I1803c201743fccf9c177a7098959083f7baf95a0
Reviewed-on: http://review.whamcloud.com/7781
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>