mm/slab: fix 'variable obj_exts set but not used' warning

slab_post_alloc_hook() uses prepare_slab_obj_exts_hook() to obtain
slabobj_ext object.  Currently the only user of slabobj_ext object in this
path is memory allocation profiling, therefore when it's not enabled this
object is not needed.  This also generates a warning when compiling with
CONFIG_MEM_ALLOC_PROFILING=n.  Move the code under this configuration to
fix the warning.  If more slabobj_ext users appear in the future, the code
will have to be changed back to call prepare_slab_obj_exts_hook().

Link: https://lkml.kernel.org/r/20240614225951.3845577-1-surenb@google.com
Fixes: 4b87369 ("mm/slab: add allocation accounting into slab allocation and free paths")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406150444.F6neSaiy-lkp@intel.com/
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Kees Cook <keescook@chromium.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: surenbaghdasaryan

mm/slub.c

@@ -3902,7 +3902,6 @@ bool slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
 			  unsigned int orig_size)
 {
 	unsigned int zero_size = s->object_size;
-	struct slabobj_ext *obj_exts;
 	bool kasan_init = init;
 	size_t i;
 	gfp_t init_flags = flags & gfp_allowed_mask;
@@ -3945,18 +3944,20 @@ bool slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
 		kmemleak_alloc_recursive(p[i], s->object_size, 1,
 					 s->flags, init_flags);
 		kmsan_slab_alloc(s, p[i], init_flags);
+#ifdef CONFIG_MEM_ALLOC_PROFILING
 		if (need_slab_obj_ext()) {
+			struct slabobj_ext *obj_exts;
+
 			obj_exts = prepare_slab_obj_exts_hook(s, flags, p[i]);
-#ifdef CONFIG_MEM_ALLOC_PROFILING
 			/*
 			 * Currently obj_exts is used only for allocation profiling.
 			 * If other users appear then mem_alloc_profiling_enabled()
 			 * check should be added before alloc_tag_add().
 			 */
 			if (likely(obj_exts))
 				alloc_tag_add(&obj_exts->ref, current->alloc_tag, s->size);
-#endif
 		}
+#endif
 	}
 
 	return memcg_slab_post_alloc_hook(s, lru, flags, size, p);