slab allocators: MAX_ORDER one off fix

MAX_ORDER is the first order that is not possible.

Use MAX_ORDER - 1 to calculate the larges possible object size in slab.h

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>