tracing: add protection around open use of current_tracer

Impact: fix to possible race conditions

There's some uses of current_tracer that is not protected by the
trace_types_lock. There is a small chance that a sysadmin changes
the tracer while the current_tracer is being referenced.

If the race is hit, it is unlikely to cause any harm since the
tracers are constant and are not freed. But some strang side
effects may occur.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>