dm: do not reuse dm_blk_ioctl block_device input as local variable

(Ab)using the @bdev passed to dm_blk_ioctl() opens the potential for
targets' .prepare_ioctl to fail if they go on to check the bdev for
!NULL.

Fixes: e56f81e0 ("dm: refactor ioctl handling")
Reported-by: NJunichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>

drivers/md/dm.c

@@ -603,9 +603,10 @@ static int dm_blk_ioctl(struct block_device *bdev, fmode_t mode,
 {
 	struct mapped_device *md = bdev->bd_disk->private_data;
 	struct dm_target *tgt;
+	struct block_device *tgt_bdev = NULL;
 	int srcu_idx, r;
 
-	r = dm_get_live_table_for_ioctl(md, &tgt, &bdev, &mode, &srcu_idx);
+	r = dm_get_live_table_for_ioctl(md, &tgt, &tgt_bdev, &mode, &srcu_idx);
 	if (r < 0)
 		return r;
 
@@ -620,7 +621,7 @@ static int dm_blk_ioctl(struct block_device *bdev, fmode_t mode,
 			goto out;
 	}
 
-	r =  __blkdev_driver_ioctl(bdev, mode, cmd, arg);
+	r =  __blkdev_driver_ioctl(tgt_bdev, mode, cmd, arg);
 out:
 	dm_put_live_table(md, srcu_idx);
 	return r;