trace: call tracer->exit only when the paird trace->enter was called

This fixes the result of graph tracer when we use other tracers
(thread_checker and/or loop_checker) at the same time.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>