ring-buffer: Revert "ring-buffer: Get timestamp after event is allocated"

The commit a4543a2f "ring-buffer: Get timestamp after event is
allocated" is needed for some future work. But after adding it, there is a
race somewhere that causes the saved timestamp to have a slight shift, and
get ahead of the actual timestamp and make it look like time goes backwards.

I'm still looking into why this happens, but in the mean time, this is
holding up other work to get in. I'm reverting the change for now (which
makes the problem go away), and will add it back after I know what is wrong
and fix it.

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