tracing: Add tracing_event_time_stamp() API

Add a tracing_event_time_stamp() API that checks if the event passed in is
not on the ring buffer but a pointer to the per CPU trace_buffered_event
which does not have its time stamp set yet.

If it is a pointer to the trace_buffered_event, then just return the
current time stamp that the ring buffer would produce.

Otherwise, return the time stamp from the event.

Link: https://lkml.kernel.org/r/20210316164114.131996180@goodmis.org



Reviewed-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>