vmalloc: fix buffer overflow in vrealloc_node_align()

Commit 4c5d3365882d ("mm/vmalloc: allow to set node and align in
vrealloc") added the ability to force a new allocation if the current
pointer is on the wrong NUMA node, or if an alignment constraint is not
met, even if the user is shrinking the allocation.

On this path (need_realloc), the code allocates a new object of 'size'
bytes and then memcpy()s 'old_size' bytes into it. If the request is to
shrink the object (size < old_size), this results in an out-of-bounds
write on the new buffer.

Fix this by bounding the copy length by the new allocation size.

Link: https://lore.kernel.org/20260420114805.3572606-2-elver@google.com
Fixes: 4c5d3365882d ("mm/vmalloc: allow to set node and align in vrealloc")
Signed-off-by: Marco Elver <elver@google.com>
Reported-by: Harry Yoo (Oracle) <harry@kernel.org>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>