SLAB: Move two remaining SLAB specific definitions to slab_def.h

Two definitions remained in slab.h that are particular to the SLAB allocator.
Move to slab_def.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>