block: use bd{grab,put}() instead of open-coding

- bd_acquire() and bd_forget() open-code bdgrab() and bdput()
- raw driver uses igrab() but never checks its return value and always
  holds another ref from bind_set() while calling it, so it's
  equivalent to bdgrab()

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/char/raw.c

@@ -71,7 +71,7 @@ static int raw_open(struct inode *inode, struct file *filp)
 	err = -ENODEV;
 	if (!bdev)
 		goto out;
-	igrab(bdev->bd_inode);
+	bdgrab(bdev);
 	err = blkdev_get(bdev, filp->f_mode | FMODE_EXCL, raw_open);
 	if (err)
 		goto out;