rcutorture: Test rcu_tasks_trace_expedite_current()

rcutorture: Test rcu_tasks_trace_expedite_current()

This commit adds a ->exp_current member to the tasks_tracing_ops structure
to test the rcu_tasks_trace_expedite_current() function.

[ paulmck: Apply kernel test robot feedback. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: bpf@vger.kernel.org
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>