ring-buffer: Allow ring_buffer_event_time_stamp() to return time stamp of all events

Currently, ring_buffer_event_time_stamp() only returns an accurate time
stamp of the event if it has an absolute extended time stamp attached to
it. To make it more robust, use the event_stamp() in case the event does
not have an absolute value attached to it.

This will allow ring_buffer_event_time_stamp() to be used in more cases
than just histograms, and it will also allow histograms to not require
including absolute values all the time.

Link: https://lkml.kernel.org/r/20210316164113.704830885@goodmis.org



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