mm: SLAB Out-of-memory diagnostics

Following the example at mm/slub.c, add out-of-memory diagnostics to the
SLAB allocator to help on debugging certain OOM conditions.

An example print out looks like this:

  <snip page allocator out-of-memory message>
  SLAB: Unable to allocate memory on node 0 (gfp=0x11200)
    cache: bio-0, object size: 192, order: 0
    node 0: slabs: 3/3, objs: 60/60, free: 0

Signed-off-by: Rafael Aquini <aquini@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>