block: mark blkdev_fsync static

blkdev_fsync is only used inside of block_dev.c since the
removal of the raw drіver.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210824151823.1575100-1-hch@lst.de


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