mm/slab_common: use SLAB_NO_MERGE instead of negative refcount

When CONFIG_MEMCG_KMEM is enabled, we disable cache merging for
KMALLOC_NORMAL caches so they don't end up merged with a cache that uses
ad-hoc __GFP_ACCOUNT when allocating. This was implemented by setting
the refcount to -1, but now we have a proper SLAB_NO_MERGE flag, so use
that instead.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>