blkdev: comment fs_holder_ops

Add a comment to @fs_holder_ops that @holder must point to a superblock.

Link: https://lore.kernel.org/r/20231024-vfs-super-freeze-v2-10-599c19f4faac@kernel.orgReviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/blkdev.h

@@ -1480,6 +1480,11 @@ struct blk_holder_ops {
 	int (*thaw)(struct block_device *bdev);
 };
 
+/*
+ * For filesystems using @fs_holder_ops, the @holder argument passed to
+ * helpers used to open and claim block devices via
+ * bd_prepare_to_claim() must point to a superblock.
+ */
 extern const struct blk_holder_ops fs_holder_ops;
 
 /*