LU-5577 obdclass: change loop indexes to unsigned

LU-5577 obdclass: change loop indexes to unsigned

Cleanup warnings about comparison between signed and unsigned.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: I2d94940251f639942142d54a561225daa8cd8a74
Reviewed-on: http://review.whamcloud.com/12387
Tested-by: Jenkins
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>