trace-cmd: Setting plugin to 'nop' clears data before it's recorded

commit e09a5db1a929ab668c273b87c4f0a32b81e1c21a
[trace-cmd: Add trace-cmd record --date option]

moved the call to disable_all() in trace_record() from after record_data()
to before it. Unfortunately disable_all() sets 'nop' in 'current_tracer'
which has the side affect of clearing 'trace', thus all the latency tracer
reports are empty/useless. Here we make disable_all() optionally call
set_plugin() thus, where we need to, we can delay the disabling of the tracer
until we have had a chance to capture 'trace'. We have added this delayed
behavior to trace_record() to fix the latency reports, for all other calls to
disable_all() we continue to have set_plugin() called.

Link: http://lkml.kernel.org/r/1333899525-6436-1-git-send-email-mark.asselstine@windriver.com

Signed-off-by: Mark Asselstine <mark.asselstine@windriver.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>