tracing: Pass the ftrace_file to the buffer lock reserve code

Pass the struct ftrace_event_file *ftrace_file to the
trace_event_buffer_lock_reserve() (new function that replaces the
trace_current_buffer_lock_reserver()).

The ftrace_file holds a pointer to the trace_array that is in use.
In the case of multiple buffers with different trace_arrays, this
allows different events to be recorded into different buffers.

Also fixed some of the stale comments in include/trace/ftrace.h

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