nfp: check idx is -ENOSPC before using it is an index

idx can be returned as -ENOSPC, so we should check for this first
before using it as an index into nn->vxlan_usecnt[] to avoid an
out of bounds array offset read.
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Acked-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/netronome/nfp/nfp_net_common.c

@@ -2578,7 +2578,7 @@ static void nfp_net_del_vxlan_port(struct net_device *netdev,
 		return;
 
 	idx = nfp_net_find_vxlan_idx(nn, ti->port);
-	if (!nn->vxlan_usecnt[idx] || idx == -ENOSPC)
+	if (idx == -ENOSPC || !nn->vxlan_usecnt[idx])
 		return;
 
 	if (!--nn->vxlan_usecnt[idx])