LU-5417 lod: fix comparison between signed and unsigned

LU-5417 lod: fix comparison between signed and unsigned

Make few members/variables related to size or indexes as unsigned.
Fix incorrect comparison between signed and unsigned variables.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: I1bb3dff189e5ad4f37c7696ffd51832af60bd8f1
Reviewed-on: http://review.whamcloud.com/11316
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>