ring-buffer/tracing: Have iterator acknowledge dropped events

Have the ring_buffer_iterator set a flag if events were dropped as it were
to go and peek at the next event. Have the trace file display this fact if
it happened with a "LOST EVENTS" message.

Link: http://lkml.kernel.org/r/20200317213417.045858900@goodmis.org



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