KFENCE: cleanup kfence_guarded_alloc() after CONFIG_SLAB removal

Some struct slab fields are initialized differently for SLAB and SLUB so
we can simplify with SLUB being the only remaining allocator.

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Marco Elver <elver@google.com>
Acked-by: David Rientjes <rientjes@google.com>
Tested-by: David Rientjes <rientjes@google.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

mm/kfence/core.c

@@ -463,11 +463,7 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
 	/* Set required slab fields. */
 	slab = virt_to_slab((void *)meta->addr);
 	slab->slab_cache = cache;
-#if defined(CONFIG_SLUB)
 	slab->objects = 1;
-#elif defined(CONFIG_SLAB)
-	slab->s_mem = addr;
-#endif
 
 	/* Memory initialization. */
 	set_canary(meta);