This keeps xfs_lowbit64 as it was since there aren't good generic helpers
there ... Patch inspired by Andi Kleen.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31472a
Signed-off-by: NDavid Chinner <david@fromorbit.com>
Signed-off-by: NEric Sandeen <sandeen@sandeen.net>
Signed-off-by: NDonald Douwsma <donaldd@sgi.com>
Signed-off-by: NLachlan McIlroy <lachlan@sgi.com>