srcu: Create an rcu_tasks_trace_expedite_current() function

srcu: Create an rcu_tasks_trace_expedite_current() function

This commit creates an rcu_tasks_trace_expedite_current() function
that expedites the current (and possibly the next) RCU Tasks Trace
grace period.

If the current RCU Tasks Trace grace period is already waiting, that wait
will complete before the expediting takes effect.  If there is no RCU
Tasks Trace  grace period in flight, this function might well create one.

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>