xsk: Fix refcount warning in xp_dma_map

Fix a potential refcount warning that a zero value is increased to one
in xp_dma_map, by initializing the refcount to one to start with,
instead of zero plus a refcount_inc().

Fixes: 921b686 ("xsk: Enable sharing of dma mappings")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/1600095036-23868-1-git-send-email-magnus.karlsson@gmail.com

net/xdp/xsk_buff_pool.c

@@ -296,7 +296,7 @@ static struct xsk_dma_map *xp_create_dma_map(struct device *dev, struct net_devi
 	dma_map->dev = dev;
 	dma_map->dma_need_sync = false;
 	dma_map->dma_pages_cnt = nr_pages;
-	refcount_set(&dma_map->users, 0);
+	refcount_set(&dma_map->users, 1);
 	list_add(&dma_map->list, &umem->xsk_dma_list);
 	return dma_map;
 }
@@ -369,7 +369,6 @@ static int xp_init_dma_info(struct xsk_buff_pool *pool, struct xsk_dma_map *dma_
 	pool->dev = dma_map->dev;
 	pool->dma_pages_cnt = dma_map->dma_pages_cnt;
 	pool->dma_need_sync = dma_map->dma_need_sync;
-	refcount_inc(&dma_map->users);
 	memcpy(pool->dma_pages, dma_map->dma_pages,
 	       pool->dma_pages_cnt * sizeof(*pool->dma_pages));
 
@@ -390,6 +389,7 @@ int xp_dma_map(struct xsk_buff_pool *pool, struct device *dev,
 		if (err)
 			return err;
 
+		refcount_inc(&dma_map->users);
 		return 0;
 	}