rcu-tasks: Scan running tasks for RCU Tasks Trace readers

rcu-tasks: Scan running tasks for RCU Tasks Trace readers

A running task might be within an RCU Tasks Trace read-side critical
section for any length of time, but will not be placed on any of the
per-CPU rcu_tasks_percpu structure's ->rtp_blkd_tasks lists.  Therefore
any RCU Tasks Trace grace-period processing that does not scan the full
task list must interact with the running tasks.

This commit therefore causes the rcu_tasks_trace_pregp_step() function
to IPI each CPU in order to place the corresponding task on the holdouts
list and to record whether or not it was in an RCU Tasks Trace read-side
critical section.  Yes, it is possible to avoid adding it to that list
if it is not a reader, but that would prevent the system from remembering
that this task was in a quiescent state.  Which is why the running tasks
are unconditionally added to the holdout list.

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>