stat: stop triggerring division by zero on bandwidth lower than 1KBps

stat: stop triggerring division by zero on bandwidth lower than 1KBps

Before the integer arithmetic is performed to obtain
percentage of aggregation this conversion of rs->agg[ddir] into KBytes
could potentially turn out to be 0.

Adjust stat.c to no longer trigger division by zero on bandwidths
greater than 0 but less than 1kilobyte a second. The fact that this
is performed on multiple occasions, a function is preferred

Signed-off-by: sribs <sribs@microsoft.com>