kill_bdev does not have any external user, so make it static.
Signed-off-by: NZheng Bin <zhengbin13@huawei.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NBart Van Assche <bvanassche@acm.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>