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 <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/slub_def.h

···
 	if (index == 0)
 		return NULL;
 
-	if (index < 0) {
+	/*
+	 * This function only gets expanded if __builtin_constant_p(size), so
+	 * testing it here shouldn't be needed.  But some versions of gcc need
+	 * help.
+	 */
+	if (__builtin_constant_p(size) && index < 0) {
 		/*
 		 * Generate a link failure. Would be great if we could
 		 * do something to stop the compile here.