[XFS] cleanup defintions of BMBT_*BITLEN macros

The BMBT_*BITLEN are currently defined in a complicated way depending on
XFS_NATIVE_HOST. But if all the macros are expanded they (obviously)
expand to the same value for both cases.

This patch defines the macros in the most simple way and updates the
comment describing them to remove outdated bits.

SGI-PV: 968563
SGI-Modid: xfs-linux-melb:xfs-kern:29320a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>