printk: nbcon: Check for device_{lock,unlock} callbacks

These callbacks are necessary to synchronize ->write_thread callback
against other operations using the same device.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Link: https://patch.msgid.link/20251208-nbcon-device-cb-fix-v2-1-36be8d195123@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>

Author: marcosps

kernel/printk/nbcon.c

@@ -1760,9 +1760,12 @@ bool nbcon_alloc(struct console *con)
 	/* Synchronize the kthread start. */
 	lockdep_assert_console_list_lock_held();
 
-	/* The write_thread() callback is mandatory. */
-	if (WARN_ON(!con->write_thread))
+	/* Check for mandatory nbcon callbacks. */
+	if (WARN_ON(!con->write_thread ||
+		    !con->device_lock ||
+		    !con->device_unlock)) {
 		return false;
+	}
 
 	rcuwait_init(&con->rcuwait);
 	init_irq_work(&con->irq_work, nbcon_irq_work);