SLUB: Simplify dma index -> size calculation

There is no need to caculate the dma slab size ourselves. We can simply
lookup the size of the corresponding non dma slab.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>