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;

fs/block_dev.c

@@ -696,7 +696,7 @@ static struct block_device *bd_acquire(struct inode *inode)
 	spin_lock(&bdev_lock);
 	bdev = inode->i_bdev;
 	if (bdev) {
-		ihold(bdev->bd_inode);
+		bdgrab(bdev);
 		spin_unlock(&bdev_lock);
 		return bdev;
 	}
@@ -712,7 +712,7 @@ static struct block_device *bd_acquire(struct inode *inode)
 			 * So, we can access it via ->i_mapping always
 			 * without igrab().
 			 */
-			ihold(bdev->bd_inode);
+			bdgrab(bdev);
 			inode->i_bdev = bdev;
 			inode->i_mapping = bdev->bd_inode->i_mapping;
 			list_add(&inode->i_devices, &bdev->bd_inodes);
@@ -735,7 +735,7 @@ void bd_forget(struct inode *inode)
 	spin_unlock(&bdev_lock);
 
 	if (bdev)
-		iput(bdev->bd_inode);
+		bdput(bdev);
 }
 
 /**