xfs: kill xfs_bmbt_rec_32/64 types

For a long time we've always stored bmap btree records in the 64bit format,
so kill off the dead 32bit type, and make sure the 64bit type is named just
xfs_bmbt_rec everywhere, without any size postfix.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>