slub: Deal with hyperthetical case of PAGE_SIZE > 2M

kmalloc_index() currently returns -1 if the PAGE_SIZE is larger than 2M
which seems to cause some concern since the callers do not check for -1.

Insert a BUG() and add a comment to the -1 explaining that the code
cannot be reached.

Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>