[netstat]: Fix for negative output values of counters after clear counters operation (sonic-net#697)

Immediately after a clear counter operation, the difference between new
counter and old counter is negative. Returning 0 in this situation

Author: lyndonsiao

utilities_common/netstat.py

@@ -13,7 +13,7 @@ def ns_diff(newstr, oldstr):
         return STATUS_NA
     else:
         new, old = int(newstr), int(oldstr)
-        return '{:,}'.format(new - old)
+        return '{:,}'.format(max(0, new - old))
 
 def ns_brate(newstr, oldstr, delta):
     """