rcu-tasks: Idle tasks on offline CPUs are in quiescent states

rcu-tasks: Idle tasks on offline CPUs are in quiescent states

commit 5c9a9ca44fda41c5e82f50efced5297a9c19760d upstream.

Any idle task corresponding to an offline CPU is in an RCU Tasks Trace
quiescent state.  This commit causes rcu_tasks_trace_postscan() to ignore
idle tasks for offline CPUs, which it can do safely due to CPU-hotplug
operations being disabled.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: KP Singh <kpsingh@kernel.org>
Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>