dm: fix opening log and cow devices for read only tables

If a table is read-only, also open any log and cow devices it uses read-only.

Previously, even read-only devices were opened read-write internally.
After patch 75f1dc0d
  block: check bdev_read_only() from blkdev_get()
was applied, loading such tables began to fail.  The patch
was reverted by e51900f7
  block: revert block_dev read-only check
but this patch fixes this part of the code to work with the original patch.
Signed-off-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm-log.c

@@ -543,7 +543,7 @@ static int disk_ctr(struct dm_dirty_log *log, struct dm_target *ti,
 		return -EINVAL;
 	}
 
-	r = dm_get_device(ti, argv[0], FMODE_READ | FMODE_WRITE, &dev);
+	r = dm_get_device(ti, argv[0], dm_table_get_mode(ti->table), &dev);
 	if (r)
 		return r;
 

drivers/md/dm-snap.c

@@ -1080,7 +1080,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	argv++;
 	argc--;
 
-	r = dm_get_device(ti, cow_path, FMODE_READ | FMODE_WRITE, &s->cow);
+	r = dm_get_device(ti, cow_path, dm_table_get_mode(ti->table), &s->cow);
 	if (r) {
 		ti->error = "Cannot get COW device";
 		goto bad_cow;