ring-buffer: have benchmark test handle discarded events

ring-buffer: have benchmark test handle discarded events

With the addition of commit:

  cdf34b881e5533e6eabc25ba96118bdcee9ddd7a
  ring-buffer: prevent adding write in discarded area

The ring buffer may now add discarded events when a write passes
the end of a buffer page. Before, a discarded event was only added
when the tracer deliberately created one. The ring buffer benchmark
test does not handle discarded events when it reads the buffer and
fails when it encounters one.

Also fix the increment for large data entries (luckily, the test did
not add any yet).

[ Impact: fix false failure of ring buffer self test ]

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