virtio-blk: virtqueue_kick() must be ordered with other virtqueue operations

It isn't safe to call it without holding the vblk->vq_lock.

Reported-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Shaohua Li <shli@fusionio.com>

Fixed another condition of virtqueue_kick() not holding the lock.

Signed-off-by: Jens Axboe <axboe@kernel.dk>