rcu-tasks: Untrack blocked RCU Tasks Trace at reader end

This commit causes rcu_read_unlock_trace() to check for the current
task being on a per-CPU list within the rcu_tasks_percpu structure,
and removes it from that list if so.  This has the effect of curtailing
tracking of a task that blocked within an RCU Tasks Trace read-side
critical section once it exits that critical section.

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>