LU-12013 lfsck: use correct buffer

LU-12013 lfsck: use correct buffer

lmm is used as a temporary pointer to structure, it can get moved within
the buffer while @size remain the same. this may cause invalid memory
access.

Change-Id: Iecc51e8bb75c678e7d8287b3798afbab8bfd1485
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/34901
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>