Revert "TTY: call tty_driver_lookup_tty unconditionally"

This reverts commit 631180ac.

It caused problems when /dev/tty is a pty:
	https://lkml.org/lkml/2011/10/12/401

Cc: Jiri Slaby <jslaby@suse.cz>
Cc: stable <stable@vger.kernel.org>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Cc: Alan Cox <alan@redhat.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/tty/tty_io.c

@@ -1821,7 +1821,7 @@ int tty_release(struct inode *inode, struct file *filp)
 
 static int tty_open(struct inode *inode, struct file *filp)
 {
-	struct tty_struct *tty;
+	struct tty_struct *tty = NULL;
 	int noctty, retval;
 	struct tty_driver *driver;
 	int index;
@@ -1892,14 +1892,17 @@ static int tty_open(struct inode *inode, struct file *filp)
 		return -ENODEV;
 	}
 got_driver:
-	/* check whether we're reopening an existing tty */
-	tty = tty_driver_lookup_tty(driver, inode, index);
-	if (IS_ERR(tty)) {
-		tty_unlock();
-		mutex_unlock(&tty_mutex);
-		tty_driver_kref_put(driver);
-		tty_free_file(filp);
-		return PTR_ERR(tty);
+	if (!tty) {
+		/* check whether we're reopening an existing tty */
+		tty = tty_driver_lookup_tty(driver, inode, index);
+
+		if (IS_ERR(tty)) {
+			tty_unlock();
+			mutex_unlock(&tty_mutex);
+			tty_driver_kref_put(driver);
+			tty_free_file(filp);
+			return PTR_ERR(tty);
+		}
 	}
 
 	if (tty) {