xfs: use accessor functions for bitmap words

Create get and set functions for rtbitmap words so that we can redefine
the ondisk format with a specific endianness.  Note that this requires
the definition of a distinct type for ondisk rtbitmap words so that the
compiler can perform proper typechecking as we go back and forth.

In the upcoming rtgroups feature, we're going to fix the problem that
rtwords are written in host endian order, which means we'll need the
distinct rtword/rtword_raw types.

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