bpf, devmap: Convert remaining READ_ONCE() to rcu_dereference_check()

There were a couple of READ_ONCE()-invocations left-over by the devmap
RCU conversion. Convert these to rcu_dereference_check() as well to avoid
complaints from sparse.

Fixes: 782347b6 ("xdp: Add proper __rcu annotations to redirect map entries")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210629093907.573598-1-toke@redhat.com

kernel/bpf/devmap.c

@@ -558,7 +558,8 @@ int dev_map_enqueue_multi(struct xdp_buff *xdp, struct net_device *dev_rx,
 
 	if (map->map_type == BPF_MAP_TYPE_DEVMAP) {
 		for (i = 0; i < map->max_entries; i++) {
-			dst = READ_ONCE(dtab->netdev_map[i]);
+			dst = rcu_dereference_check(dtab->netdev_map[i],
+						    rcu_read_lock_bh_held());
 			if (!is_valid_dst(dst, xdp, exclude_ifindex))
 				continue;
 
@@ -654,7 +655,8 @@ int dev_map_redirect_multi(struct net_device *dev, struct sk_buff *skb,
 
 	if (map->map_type == BPF_MAP_TYPE_DEVMAP) {
 		for (i = 0; i < map->max_entries; i++) {
-			dst = READ_ONCE(dtab->netdev_map[i]);
+			dst = rcu_dereference_check(dtab->netdev_map[i],
+						    rcu_read_lock_bh_held());
 			if (!dst || dst->dev->ifindex == exclude_ifindex)
 				continue;