xfs: introduce realtime refcount btree ondisk definitions

xfs: introduce realtime refcount btree ondisk definitions

Add the ondisk structure definitions for realtime refcount btrees. The
realtime refcount btree will be rooted from a hidden inode so it needs
to have a separate btree block magic and pointer format.

Next, add everything needed to read, write and manipulate refcount btree
blocks. This prepares the way for connecting the btree operations
implementation, though the changes to actually root the rtrefcount btree
in an inode come later.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>