xfs_ialloc_btree.h has a a cuple of macros that only obsfucate the code
but don't provide any abstraction benefits.  This patches removes those
and cleans up the reamaining defintions up a little.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>