tty: usb-serial krefs

Use kref in the USB serial drivers so that we don't free tty structures
from under the URB receive handlers as has historically been the case if
you were unlucky. This also gives us a framework for general tty drivers to
use tty_port objects and refcount.

Contains two err->dev_err changes merged together to fix clashes in the
-next tree.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/usb/serial/cypress_m8.c

@@ -1286,7 +1286,7 @@ static void cypress_read_int_callback(struct urb *urb)
 	}
 	spin_unlock_irqrestore(&priv->lock, flags);
 
-	tty = port->port.tty;
+	tty = tty_port_tty_get(&port->port);
 	if (!tty) {
 		dbg("%s - bad tty pointer - exiting", __func__);
 		return;
@@ -1362,7 +1362,7 @@ static void cypress_read_int_callback(struct urb *urb)
 					data[i]);
 			tty_insert_flip_char(tty, data[i], tty_flag);
 		}
-		tty_flip_buffer_push(port->port.tty);
+		tty_flip_buffer_push(tty);
 	}
 
 	spin_lock_irqsave(&priv->lock, flags);
@@ -1371,6 +1371,7 @@ static void cypress_read_int_callback(struct urb *urb)
 	spin_unlock_irqrestore(&priv->lock, flags);
 
 continue_read:
+	tty_kref_put(tty);
 
 	/* Continue trying to always read... unless the port has closed. */