slab: Fix !NUMA config for CONFIG_TRACING

CONFIG_TRACING with !CONFIG_NUMA does not work right since we simply
define a function that is not provided by the allocations.

We need to fallback to a special inline functions for
kmem_cache_alloc_node_trace that ignores the node information even if
CONFIG_TRACING is set.

For that to work move the non NUMA trace definitions in to the #ifdefs
earlier in slab.h.

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