mm/slub: initialize the free pointer of the bootstrap kmem_cache_node

With slub_debug=FZPU, validating kmem_cache_node reports

  BUG kmem_cache_node (Not tainted): Freepointer corrupt
  Object 0xffff888000140040 @offset=64 fp=0x5a5a5a5a5a5a5a5a

and taints the kernel. Poisoning places the bootstrap object's free
pointer outside the object, where check_object() checks it even while
allocated.

early_kmem_cache_node_alloc() previously took this object from a
freelist with an initialized pointer. It now takes it from the slab
iterator and builds the freelist only for the remaining objects,
leaving its pointer filled with POISON_INUSE.

Initialize the bootstrap object's free pointer to NULL.

Fixes: dc795d4c02 ("mm/slub: defer freelist construction until after bulk allocation from a new slab")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: Harry Yoo (Meta) <harry@kernel.org>
Link: https://patch.msgid.link/20261010203906.93156-1-kmehltretter@gmail.com
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Shengming Hu <hu.shengming@zte.com.cn>
Tested-by: David Rientjes <rientjes@google.com>
Signed-off-by: Harry Yoo (Meta) <harry@kernel.org>

mm/slub.c

@@ -7891,6 +7891,7 @@ static void early_kmem_cache_node_alloc(int node)
 
 	n = next_slab_obj(kmem_cache_node, &iter);
 	BUG_ON(!n);
+	set_freepointer(kmem_cache_node, n, NULL);
 
 	slab->inuse = 1;
 	build_slab_freelist(kmem_cache_node, slab, &iter);