TTY: check if tty->port is assigned

And if not, complain loudly. None in-kernel module should trigger
that, but let us find out for sure. On the other hand, all the
out-of-tree modules will hit that. Give them some time (maybe one
release) to catch up.

Signed-off-by: Jiri Slaby <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/tty_io.c

···
 	if (!tty->port)
 		tty->port = driver->ports[idx];
 
+	WARN_RATELIMIT(!tty->port,
+			"%s: %s driver does not set tty->port. This will crash the kernel later. Fix the driver!\n",
+			__func__, tty->driver->name);
+
 	/*
 	 * Structures all installed ... call the ldisc open routines.
 	 * If we fail here just call release_tty to clean up.  No need