[XFS] Pick a single default inode cluster size.

Remove scaling of inode "clusters" based on machine memory; small cluster
cut-point was an unrealistic 32MB and was probably never tested.

Removes another user of xfs_physmem.

SGI-PV: 968563
SGI-Modid: xfs-linux-melb:xfs-kern:29324a

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>