SLUB: SLUB_DEBUG must depend on SLUB

Otherwise people get asked about SLUB_DEBUG even if they have another
slab allocator enabled.

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>