xfs: use accessor functions for summary info words

Create get and set functions for rtsummary 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 summary info words so that
the compiler can perform proper typechecking.

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