xfs: define the on-disk realtime refcount btree format

xfs: define the on-disk realtime refcount btree format

Start filling out the rtrefcount btree implementation. Start with the
on-disk btree format; 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>