xfs: introduce realtime refcount btree ondisk definitions

xfs: introduce realtime refcount btree ondisk definitions

Source kernel commit: 9abe03a0e4f978615a2b1b484b8d09ca84c16ea0

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>