tracing: Only calculate stats of tracepoint benchmarks for 2^32 times

When calculating the average and standard deviation, it is required that
the count be less than UINT_MAX, otherwise the do_div() will get
undefined results. After 2^32 counts of data, the average and standard
deviation should pretty much be set anyway.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>