mlx4: fix race accessing page->_count

This is illegal to use atomic_set(&page->_count, ...) even if we 'own'
the page. Other entities in the kernel need to use get_page_unless_zero()
to get a reference to the page before testing page properties, so we could
loose a refcount increment.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_rx.c

@@ -76,10 +76,10 @@ static int mlx4_alloc_pages(struct mlx4_en_priv *priv,
 	page_alloc->dma = dma;
 	page_alloc->page_offset = frag_info->frag_align;
 	/* Not doing get_page() for each frag is a big win
-	 * on asymetric workloads.
+	 * on asymetric workloads. Note we can not use atomic_set().
 	 */
-	atomic_set(&page->_count,
-		   page_alloc->page_size / frag_info->frag_stride);
+	atomic_add(page_alloc->page_size / frag_info->frag_stride - 1,
+		   &page->_count);
 	return 0;
 }