tracing/ftrace: fix a race condition in sched_switch tracer

Impact: fix race condition in sched_switch tracer

This patch fixes a race condition in the sched_switch tracer. If
several tasks (IE: concurrent initcalls) are playing with
tracing_start_cmdline_record() and tracing_stop_cmdline_record(), the
following situation could happen:

_ Task A and B are using the same tracepoint probe. Task A holds it.
  Task B is sleeping and doesn't hold it.

_ Task A frees the sched tracer, then sched_ref is decremented to 0.

_ Task A is preempted and hadn't yet unregistered its tracepoint
  probe, then B runs.

_ B increments sched_ref, sees it's 1 and then guess it has to
  register its probe. But it has not been freed by task A.

_ A lot of bad things can happen after that...

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>