Fix code / doc about check_botnet_pings()

[michaelortmann]

Found by: michaelortmann Patch by: michaelortmann Fixes:

One-line summary: Fix code/doc about check_botnet_pings()

Additional description (if needed): code / doc was wrong about check_botnet_pings(). Its called every 5 minutes. Also lets just use modulo operation.

Test cases demonstrating functionality (if applicable): No functional change