LU-7085 lov: trying smaller memory allocations

LU-7085 lov: trying smaller memory allocations

Reduce struct lov_io_sub to smaller memory usage
on wide-stripe file systems.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Change-Id: Ie92cec079f6514efca71663af1ae6ae1e34c73cb
Reviewed-on: http://review.whamcloud.com/17476
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>