[XFS] Fix build regression from mod/commit which did cleanup of xfs_bmbt_*set_allf

In sgi mod# xfs-linux-melb:xfs-kern:29319a, the variable renaming was not
complete and variable 'b' was left unchanged for non-lbd 32 bit machines.

SGI-PV: 968563
SGI-Modid: xfs-linux-melb:xfs-kern:29469a

Signed-off-by: Michal Piotrowski <michal.k.k.piotrowski@gmail.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>