aio: nullify aio->ring_pages after freeing it

After freeing ring_pages we leave it as is causing a dangling pointer. This
has already caused an issue so to help catching any issues in the future
NULL it out.
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>

fs/aio.c

@@ -196,8 +196,10 @@ static void aio_free_ring(struct kioctx *ctx)
 
 	put_aio_ring_file(ctx);
 
-	if (ctx->ring_pages && ctx->ring_pages != ctx->internal_pages)
+	if (ctx->ring_pages && ctx->ring_pages != ctx->internal_pages) {
 		kfree(ctx->ring_pages);
+		ctx->ring_pages = NULL;
+	}
 }
 
 static int aio_ring_mmap(struct file *file, struct vm_area_struct *vma)