mon: osd df: fix average_util calculation

mon: osd df: fix average_util calculation

average_util should be calculated from the same set of nodes that
dump_item considers when calculating the stddev.

While here, remove unnecessary check for kb > 0:
get_bucket_utilization returns false if kb <= 0.

Also, update get_bucket_utilization accordingly to the preferred
convention to use pointers and not references for output arguments.

Signed-off-by: Mykola Golub <mgolub@mirantis.com>