Btrfs: fix a bug when opening seed devices

Initialize fs_info->bdev_holder a bit earlier to be able to pass a
correct holder id to blkdev_get() when opening seed devices with O_EXCL.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/btrfs/super.c

@@ -939,14 +939,14 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 
 		s->s_flags = flags | MS_NOSEC;
 		strlcpy(s->s_id, bdevname(bdev, b), sizeof(s->s_id));
+		btrfs_sb(s)->fs_info->bdev_holder = fs_type;
 		error = btrfs_fill_super(s, fs_devices, data,
 					 flags & MS_SILENT ? 1 : 0);
 		if (error) {
 			deactivate_locked_super(s);
 			return ERR_PTR(error);
 		}
 
-		btrfs_sb(s)->fs_info->bdev_holder = fs_type;
 		s->s_flags |= MS_ACTIVE;
 	}