LU-8527 osd: ot_credits must be 32bit

LU-8527 osd: ot_credits must be 32bit

ot_credits used to calculate journal credits must be 32bit,
otherwise it can overflow by huge transactions (e.g. 1K striped dirs)

Change-Id: I53545989fd3653e72b5227281fc2ec6b2ccd309d
Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-on: http://review.whamcloud.com/22782
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>