rcu-tasks: Track blocked RCU Tasks Trace readers

rcu-tasks: Track blocked RCU Tasks Trace readers

This commit places any task that has ever blocked within its current
RCU Tasks Trace read-side critical section on a per-CPU list within the
rcu_tasks_percpu structure.  Tasks are removed from this list when they
exit by the exit_tasks_rcu_finish_trace() function.  The purpose of this
commit is to provide the information needed to eliminate the current
scan of the full task list.

This commit offsets the INT_MIN value for ->trc_reader_nesting with the
new nesting level in order to avoid queueing tasks that are exiting
their read-side critical sections.

[ paulmck: Apply kernel test robot feedback. ]
[ paulmck: Apply feedback from syzbot+9bb26e7c5e8e4fa7e641@syzkaller.appspotmail.com ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: syzbot <syzbot+9bb26e7c5e8e4fa7e641@syzkaller.appspotmail.com>
Tested-by: "Zhang, Qiang1" <qiang1.zhang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.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>