tty: fix switching consoles during suspend

[Arnd Bergmann <arnd@arndb.de>]

vt_waitactive now expects to be called with the big tty mutex, which
it tries to release before going to sleep.

This fixes the only caller that does not hold the BTM already.
---

Greg, please apply to the tty series or fold into
"tty: introduce wait_event_interruptible_tty", which introduces
the problem.

diff --git a/drivers/char/vt_ioctl.c b/drivers/char/vt_ioctl.c
index 45410c8..2bbeaae 100644
--- a/drivers/char/vt_ioctl.c
+++ b/drivers/char/vt_ioctl.c
@@ -1761,10 +1761,13 @@ int vt_move_to_console(unsigned int vt, int alloc)
 		return -EIO;
 	}
 	release_console_sem();
+	tty_lock();
 	if (vt_waitactive(vt + 1)) {
 		pr_debug("Suspend: Can't switch VCs.");
+		tty_unlock();
 		return -EINTR;
 	}
+	tty_unlock();
 	return prev;
 }