net/mlx5e: Fix use-after-free in mlx5e_stats_grp_sw_update_stats

We need to sync page pool stats only for active channels. Reading ethtool
stats on a down netdev or a netdev with modified number of channels will
result in a user-after-free, trying to access page pools that are freed
already.

BUG: KASAN: use-after-free in mlx5e_stats_grp_sw_update_stats+0x465/0xf80
Read of size 8 at addr ffff888004835e40 by task ethtool/720

Fixes: cc10e84b ("mlx5: add support for page_pool_get_stats")
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Reported-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Joe Damato <jdamato@fastly.com>
Link: https://lore.kernel.org/r/20220312005353.786255-1-saeed@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mellanox/mlx5/core/en_stats.c

@@ -521,14 +521,15 @@ static MLX5E_DECLARE_STATS_GRP_OP_UPDATE_STATS(sw)
 
 	memset(s, 0, sizeof(*s));
 
+	for (i = 0; i < priv->channels.num; i++) /* for active channels only */
+		mlx5e_stats_update_stats_rq_page_pool(priv->channels.c[i]);
+
 	for (i = 0; i < priv->stats_nch; i++) {
 		struct mlx5e_channel_stats *channel_stats =
 			priv->channel_stats[i];
 
 		int j;
 
-		mlx5e_stats_update_stats_rq_page_pool(priv->channels.c[i]);
-
 		mlx5e_stats_grp_sw_update_stats_rq_stats(s, &channel_stats->rq);
 		mlx5e_stats_grp_sw_update_stats_xdpsq(s, &channel_stats->rq_xdpsq);
 		mlx5e_stats_grp_sw_update_stats_ch_stats(s, &channel_stats->ch);