fix page number calculation bug for block layout decode buffer

Signed-off-by: Jim Rees <rees@umich.edu>
Suggested-by: Andy Adamson <andros@netapp.com>
Suggested-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>