block: move policy from disk to part0

Move disk->policy to part0->policy.  Implement and use get_disk_ro().

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fs/partitions/check.c b/fs/partitions/check.c
index 24d2c56..ace6d03 100644
--- a/fs/partitions/check.c
+++ b/fs/partitions/check.c
@@ -375,7 +375,7 @@
 	p->start_sect = start;
 	p->nr_sects = len;
 	p->partno = partno;
-	p->policy = disk->policy;
+	p->policy = get_disk_ro(disk);
 
 	dname = dev_name(ddev);
 	if (isdigit(dname[strlen(dname) - 1]))