xfs: convert the rtbitmap block and bit macros to static inline functions

Replace these macros with typechecked helper functions.  Eventually
we're going to add more logic to the helpers and it'll be easier if we
don't have to macro it up.

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