LU-5577 obdclass: change uuid_unpack arg to size_t

LU-5577 obdclass: change uuid_unpack arg to size_t

Cleanup warnings about comparison between signed and unsigned.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: Ib577a36879a1a57f20f13a5c4c697ba404e113fa
Reviewed-on: http://review.whamcloud.com/12389
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>