Move the policy field to struct block_device and rename it to the
more descriptive bd_read_only.  Also turn the field into a bool as it
is used as such.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJan Kara <jack@suse.cz>
Reviewed-by: NHannes Reinecke <hare@suse.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>