[XFS] Fix xfs_lowbit64

xfs_lowbit64 was broken on 32 bit platforms in a recent cleanup of the xfs
bitops. Fix it back up again.

SGI-PV: 974005
SGI-Modid: xfs-linux-melb:xfs-kern:30202a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>