LU-2127 osc: fix wrong osc_announce_cached() msgs

LU-2127 osc: fix wrong osc_announce_cached() msgs

In osc_announce_cached() wrong signed/unsigned comparison has been fixed with
a cast.

Unlikely conditions have also been specified, along with some changes to comply
coding rules.

Signed-off-by: Bruno Faccini <bfaccini62@gmail.com>
Change-Id: I8f8f2bdfc673d31054b4db2063d32c3dc34100d8
Reviewed-on: http://review.whamcloud.com/4246
Reviewed-by: Keith Mannthey <keith.mannthey@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>