xfs_{db,repair}: use accessor functions for bitmap words

xfs_{db,repair}: use accessor functions for bitmap words

Port xfs_db and xfs_repair to use 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>
Reviewed-by: Bill O'Donnell <bodonnel@redhat.com>