rcu-tasks: Use more aggressive polling for RCU Tasks Trace

The RCU Tasks Trace grace periods are too slow, as in 40x slower than
those of RCU Tasks.  This is due to my having assumed a one-second grace
period was OK, and thus not having optimized any further.  This commit
provides the first step in this optimization process, namely by allowing
the task_list scan backoff interval to be specified on a per-flavor basis,
and then speeding up the scans for RCU Tasks Trace.  However, kernels
built with CONFIG_TASKS_TRACE_RCU_READ_MB=y continue to use the old slower
backoff, consistent with that Kconfig option's goal of reducing IPIs.

Link: https://lore.kernel.org/bpf/CAADnVQK_AiX+S_L_A4CQWT11XyveppBbQSQgH_qWGyzu_E8Yeg@mail.gmail.com/


Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>