LU-2991 osd: Overflow of transaction credits counters

LU-2991 osd: Overflow of transaction credits counters

Switching size of transaction credits counters from uchar to
ushort to avoid possible overflow scenarios, like when
wide-striping.
Also allow OSD_TRACK_DECLARES to be undefined without
compile-time errors for unsatisfied-externals/unused vars.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I2588cf11741ca4e3ee80b795a7d4318f9ed4fd3d
Reviewed-on: http://review.whamcloud.com/5830
Tested-by: Hudson
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>