tracing: Call traceoff trigger after event is recorded

Call traceoff trigger after the event is recorded.
Since current traceoff trigger is called before recording
the event, we can not know what event stopped tracing.

Typical usecase of traceoff/traceon trigger is tracing
function calls and trace events between a pair of events.
For example, trace function calls between syscall entry/exit.
In that case, it is useful if we can see the return code
of the target syscall.

Link: http://lkml.kernel.org/r/147335074530.12462.4526186083406015005.stgit@devbox



Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>