tty: Prevent hw state corruption in exclusive mode reopen

Exclusive mode ttys (TTY_EXCLUSIVE) do not allow further reopens;
fail the condition before associating the file pointer and calling
the driver open() method.

Prevents DTR programming when the tty is already in exclusive mode.
Reported-by: NShreyas Bethur <shreyas.bethur@ni.com>
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Acked-by: NShreyas Bethur <shreyas.bethur@ni.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -1464,6 +1464,9 @@ static int tty_reopen(struct tty_struct *tty)
 	    driver->subtype == PTY_TYPE_MASTER)
 		return -EIO;
 
+	if (test_bit(TTY_EXCLUSIVE, &tty->flags) && !capable(CAP_SYS_ADMIN))
+		return -EBUSY;
+
 	tty->count++;
 
 	WARN_ON(!tty->ldisc);
@@ -2106,10 +2109,6 @@ static int tty_open(struct inode *inode, struct file *filp)
 		retval = -ENODEV;
 	filp->f_flags = saved_flags;
 
-	if (!retval && test_bit(TTY_EXCLUSIVE, &tty->flags) &&
-						!capable(CAP_SYS_ADMIN))
-		retval = -EBUSY;
-
 	if (retval) {
 #ifdef TTY_DEBUG_HANGUP
 		printk(KERN_DEBUG "%s: error %d in opening %s...\n", __func__,