block: remove flush_disk

flush_disk has only two callers, so open code it there.  That also helps
clarifying the error message for the particular case, and allows to remove
setting bd_invalidated in check_disk_size_change, which will be cleared
again instantly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>