ring-buffer: add total count in ring-buffer-benchmark

It is nice to see the overhead of the benchmark test when tracing is
disabled. That is, we turn off the ring buffer just to see what the
cost of running the loop that calls into the ring buffer is.

Currently, if no entries wer made, we get 0. This is not informative.
This patch changes it to check if we had any "missed" (non recorded)
events. If so, a total count is also reported.

[ Impact: evaluate the over head of the ring buffer benchmark test ]

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