rcu-tasks: Provide rcu_trace_implies_rcu_gp()

As an accident of implementation, an RCU Tasks Trace grace period also
acts as an RCU grace period.  However, this could change at any time.
This commit therefore creates an rcu_trace_implies_rcu_gp() that currently
returns true to codify this accident.  Code relying on this accident
must call this function to verify that this accident is still happening.

Reported-by: Hou Tao <houtao@huaweicloud.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Link: https://lore.kernel.org/r/20221014113946.965131-2-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>