LU-12268 osd: BUG_ON for IAM corruption

LU-12268 osd: BUG_ON for IAM corruption

The patch adds strict checks of buffer head overflow
for IAM dx blocks.

HPE-bug-id: LUS-10178
Signed-off-by: Alexander Boyko <alexander.boyko@hpe.com>
Change-Id: I1608f6cbf00b5120fbc36d0c65fcfe37c43e375f
Reviewed-on: https://review.whamcloud.com/45072
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <artem.blagodarenko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>