syzbot is reporting that an unprivileged user who logged in from tty
console can crash the system using a reproducer shown below [1], for
n_hdlc_tty_wakeup() is synchronously calling n_hdlc_send_frames().

----------
  #include <sys/ioctl.h>
  #include <unistd.h>

  int main(int argc, char *argv[])
  {
    const int disc = 0xd;

    ioctl(1, TIOCSETD, &disc);
    while (1) {
      ioctl(1, TCXONC, 0);
      write(1, "", 1);
      ioctl(1, TCXONC, 1); /* Kernel panic - not syncing: scheduling while atomic */
    }
  }
----------

Linus suspected that "struct tty_ldisc"->ops->write_wakeup() must not
sleep, and Jiri confirmed it from include/linux/tty_ldisc.h. Thus, defer
n_hdlc_send_frames() from n_hdlc_tty_wakeup() to a WQ context like
net/nfc/nci/uart.c does.

Link: https://syzkaller.appspot.com/bug?extid=5f47a8cea6a12b77a876 [1]
Reported-by: syzbot <syzbot+5f47a8cea6a12b77a876@syzkaller.appspotmail.com>
Cc: stable <stable@vger.kernel.org>
Analyzed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Confirmed-by: Jiri Slaby <jirislaby@kernel.org>
Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Link: https://lore.kernel.org/r/40de8b7e-a3be-4486-4e33-1b1d1da452f8@i-love.sakura.ne.jp
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>