LU-6584 osd: prevent int type overflow in osd_read_prep()

LU-6584 osd: prevent int type overflow in osd_read_prep()

There is possible type overflow in osd_read_prep() that may
cause too big value in lnb_rc followed by assertion.

Signed-off-by: Mikhail Pershin <mike.pershin@intel.com>
Change-Id: If17b533e7d0dcae7db57eefc0e5981821f628c56
Reviewed-on: http://review.whamcloud.com/16685
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Cliff White <cliff.white@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>