ttyprintk: Add TTY hangup callback.

syzbot is reporting hung task due to flood of

  tty_warn(tty, "%s: tty->count = 1 port count = %d\n", __func__,
           port->count);

message [1], for ioctl(TIOCVHANGUP) prevents tty_port_close() from
decrementing port->count due to tty_hung_up_p() == true.

----------
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <unistd.h>

int main(int argc, char *argv[])
{
	int i;
	int fd[10];

	for (i = 0; i < 10; i++)
		fd[i] = open("/dev/ttyprintk", O_WRONLY);
	ioctl(fd[0], TIOCVHANGUP);
	for (i = 0; i < 10; i++)
		close(fd[i]);
	close(open("/dev/ttyprintk", O_WRONLY));
	return 0;
}
----------

When TTY hangup happens, port->count needs to be reset via
"struct tty_operations"->hangup callback.

[1] https://syzkaller.appspot.com/bug?id=39ea6caa479af471183997376dc7e90bc7d64a6aReported-by: syzbot <syzbot+43e93968b964e369db0b@syzkaller.appspotmail.com>
Reported-by: syzbot <syzbot+3ed715090790806d8b18@syzkaller.appspotmail.com>
Tested-by: syzbot <syzbot+43e93968b964e369db0b@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: 24b4b67d ("add ttyprintk driver")
Link: https://lore.kernel.org/r/17e0652d-89b7-c8c0-fb53-e7566ac9add4@i-love.sakura.ne.jpSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/ttyprintk.c

@@ -158,12 +158,23 @@ static int tpk_ioctl(struct tty_struct *tty,
 	return 0;
 }
 
+/*
+ * TTY operations hangup function.
+ */
+static void tpk_hangup(struct tty_struct *tty)
+{
+	struct ttyprintk_port *tpkp = tty->driver_data;
+
+	tty_port_hangup(&tpkp->port);
+}
+
 static const struct tty_operations ttyprintk_ops = {
 	.open = tpk_open,
 	.close = tpk_close,
 	.write = tpk_write,
 	.write_room = tpk_write_room,
 	.ioctl = tpk_ioctl,
+	.hangup = tpk_hangup,
 };
 
 static const struct tty_port_operations null_ops = { };