ptrace: ptrace_check_attach(): rename @kill to @ignore_state and add comments

    PTRACE_INTERRUPT is going to be added which should also skip
    task_is_traced() check in ptrace_check_attach().  Rename @kill to
    @ignore_state and make it bool.  Add function comment while at it.

    This patch doesn't introduce any behavior difference.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
ptrace: relocate set_current_state(TASK_TRACED) in ptrace_stop()

    In ptrace_stop(), after arch hook is done, the task state and jobctl
    bits are updated while holding siglock.  The ordering requirement
    there is that TASK_TRACED is set before JOBCTL_TRAPPING is cleared to
    prevent ptracer waiting on TRAPPING doesn't end up waking up TRACED is
    actually set and sees TASK_RUNNING in wait(2).

    Move set_current_state(TASK_TRACED) to the top of the block and
    reorganize comments.  This makes the ordering more obvious
    (TASK_TRACED before other updates) and helps future updates to group
    stop participation.

    This patch doesn't cause any functional change.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Signed-off-by: Oleg Nesterov <oleg@redhat.com>