[OLK-5.10] tty: fix possible deadlock in console_unlock

Syzkaller hit 'possible deadlock in console_unlock' for several times.
Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&(&port->lock)->rlock);
                               lock(&port_lock_key);
                               lock(&(&port->lock)->rlock);
  lock(console_owner);

The problem is that call_console_driver->console_driver also can do
this thing

   uart_port->lock
    tty_wakeup
     tty_port->lock

So we can have the following:

tty_write
 tty_port->lock
  printk
   call_console_driver
    console_driver
     uart_port->lock
      tty_wakeup
       tty_port->lock      << deadlock

To solve this problem,  switch to printk_safe mode around that kmalloc(),
this will redirect all printk()-s from kmalloc() to a special per-CPU
buffer, which will be flushed later from a safe context (irq work).

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If you have any questions, please contact the SIG: Kernel, and any of the maintainers: @Xie XiuQi, @YangYingliang, @成坚 (CHENG Jian).

https://lore.kernel.org/lkml/20181017044843.GD1068@jagdpanzerIV/T/

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[OLK-5.10] tty: fix possible deadlock in console_unlock

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