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.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>