The API in tree log code has done sort of changes, and it proves that
we can benifit from using token, so do the same thing here.

function_graph tracer's timer shows that it costs nearly half time
of before(39.788us -> 22.391us).
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>