slub: fix handling of oversized slabs

I'm getting zillions of undefined references to __kmalloc_size_too_large on
alpha.  For some reason alpha is building out-of-line copies of kmalloc_slab()
into lots of compilation units.

It turns out that gcc just isn't smart enough to work out that
__builtin_contant_p(size)==true implies that __builtin_contant_p(index)==true.

So let's give it a bit of help.

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