ring-buffer: Put back the length if crossed page with add_timestamp

Commit fcc742ea "ring-buffer: Add event descriptor to simplify passing
data" added a descriptor that holds various data instead of passing around
several variables through parameters. The problem was that one of the
parameters was modified in a function and the code was designed not to have
an effect on that modified  parameter. Now that the parameter is a
descriptor and any modifications to it are non-volatile, the size of the
data could be unnecessarily expanded.

Remove the extra space added if a timestamp was added and the event went
across the page.

Cc: stable@vger.kernel.org # 4.3+
Fixes: fcc742ea

 "ring-buffer: Add event descriptor to simplify passing data"
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>