ring-buffer: Pass delta by value and not by reference

The delta between events is passed to the timestamp code by reference
and the timestamp code will reset the value. But it can be reset
from the caller. No need to pass it in by reference.

By changing the call to pass by value, lets gcc optimize the code
a bit more where it can store the delta in a register and not
worry about updating the reference.

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