ring-buffer: Optimize rb_iter_head_event()

As it is fine to perform several "peeks" of event data in the ring buffer
via the iterator before moving it forward, do not re-read the event, just
return what was read before. Otherwise, it can cause inconsistent results,
especially when testing multiple CPU buffers to interleave them.

Link: http://lkml.kernel.org/r/20200317213416.592032170@goodmis.org



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