tracing: Rename ftrace_event_buffer to trace_event_buffer.

The name "ftrace" really refers to the function hook infrastructure. It
is not about the trace_events. The ftrace_event_buffer functions and data
structures are for trace_events and not for function hooks. Rename them
to trace_event_buffer*.

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