vt_ioctl: fix lock imbalance

Don't return from switch/case directly in vt_ioctl. Set ret and break
instead so that we unlock BKL.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>