PR28804: tune default stap -s ## buffer size on small RAM machines

PR28804: tune default stap -s ## buffer size on small RAM machines

Insert a forgotten division by num_online_cpu() to adjust downward the
calculated bufsize.  Tweak normal defaults back to 128 * 2 * 64K
(16MB) per CPU, as the stap man page indicates.  This may need further
tweaking when balancing against staprun consumption performance, but
at least we have the docs lined up with the code at the moment.