pty: Fix lock inversion

The ptmx_open path takes the tty and devpts locks in the wrong order
because tty_init_dev locks and returns a locked tty.  As far as I can
tell this is actually safe anyway because the tty being returned is new
so nobody can get a reference to lock it at this point.

However we don't even need the devpts lock at this point, it's only held
as a byproduct of the way the locks were pushe down.
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Acked-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/tty/pty.c

@@ -633,7 +633,6 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 	mutex_unlock(&devpts_mutex);
 
 	mutex_lock(&tty_mutex);
-	mutex_lock(&devpts_mutex);
 	tty = tty_init_dev(ptm_driver, index);
 
 	if (IS_ERR(tty)) {
@@ -643,7 +642,6 @@ static int ptmx_open(struct inode *inode, struct file *filp)
 
 	/* The tty returned here is locked so we can safely
 	   drop the mutex */
-	mutex_unlock(&devpts_mutex);
 	mutex_unlock(&tty_mutex);
 
 	set_bit(TTY_PTY_LOCK, &tty->flags); /* LOCK THE SLAVE */