tracing: fix sparse warning: attribute function with __acquires/__releases

Fix this sparse warning:

  kernel/trace/trace.c:458:9: warning: context imbalance in 'register_tracer' - unexpected unlock

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>