bdev: add freeze and thaw holder operations

Add block device freeze and thaw holder operations. Follow-up patches
will implement block device freeze and thaw based on stuct
blk_holder_ops.

Link: https://lore.kernel.org/r/20231024-vfs-super-freeze-v2-4-599c19f4faac@kernel.orgReviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/blkdev.h

@@ -1468,6 +1468,16 @@ struct blk_holder_ops {
 	 * Sync the file system mounted on the block device.
 	 */
 	void (*sync)(struct block_device *bdev);
+
+	/*
+	 * Freeze the file system mounted on the block device.
+	 */
+	int (*freeze)(struct block_device *bdev);
+
+	/*
+	 * Thaw the file system mounted on the block device.
+	 */
+	int (*thaw)(struct block_device *bdev);
 };
 
 extern const struct blk_holder_ops fs_holder_ops;