printk: Use two irq_works instead per-CPU

In commit b845b517b5e37 ("printk: robustify printk") initially got its
delayed wake up which is today implemented via the irq_work. Back then
it was a variable which was assigned to 1 if a pending wake up was
waiting. This variable was checked on each timer tick. Therefore it
makes sense to assign a per-CPU variable with this task to avoid cache
line bouncing when each CPU reads the variable on each timer tick
simultaneously.

Today we have a irq_work which is enqueued onto a list and handled if it
has work items assigned. This list is already per-CPU.

We could replace the per-CPU irq_work and the per-CPU printk_pending with
two irq_work structs: One for PRINTK_PENDING_WAKEUP and the other for
PRINTK_PENDING_OUTPUT.
While unrolling the conditions in __wake_up_klogd() it becomes obvious
that PRINTK_PENDING_WAKEUP is always passed as an argument but never
checked. The return value wq_has_sleeper() of does not really matter
it's sole purpose is to enforce the needed memory barrier.

Redo the irq_work by:
- Use one irq_work (pending_wakeup_work) for klogd wakes. Schedule it
  only if there is a sleeper waiting.
- Use one irq_work (&pending_output_work) for output printing. Schedule
  it only if PRINTK_PENDING_OUTPUT passed.
- Drop unused PRINTK_PENDING_WAKEUP.
- Remove the preempt_disable() section. Its purpose was to ensure that
  printk_pending and wake_up_klogd_work is accessed from the same CPU.

Worst case is defer_console_output() where two irq_work are scheduled but
those get processed one after the other during the HZ tick so it
shouldn't get worse.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Link: https://patch.msgid.link/20260914143412.1sMcp89V@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>

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