tracing: Make struct ring_buffer less ambiguous

As there's two struct ring_buffers in the kernel, it causes some confusion.
The other one being the perf ring buffer. It was agreed upon that as neither
of the ring buffers are generic enough to be used globally, they should be
renamed as:

   perf's ring_buffer -> perf_buffer
   ftrace's ring_buffer -> trace_buffer

This implements the changes to the ring buffer that ftrace uses.

Link: https://lore.kernel.org/r/20191213140531.116b3200@gandalf.local.home

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

kernel/trace/trace_hwlat.c

@@ -104,7 +104,7 @@ static void trace_hwlat_sample(struct hwlat_sample *sample)
 {
 	struct trace_array *tr = hwlat_trace;
 	struct trace_event_call *call = &event_hwlat;
-	struct ring_buffer *buffer = tr->array_buffer.buffer;
+	struct trace_buffer *buffer = tr->array_buffer.buffer;
 	struct ring_buffer_event *event;
 	struct hwlat_entry *entry;
 	unsigned long flags;