It isn't safe to call it without holding the vblk->vq_lock.
Reported-by: NDave Chinner <david@fromorbit.com>
Signed-off-by: NShaohua Li <shli@fusionio.com>

Fixed another condition of virtqueue_kick() not holding the lock.
Signed-off-by: NJens Axboe <axboe@kernel.dk>