add an ebpf offload tethering statistics map

Test: build, atest
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I1c3b2f2d61bd139f14548e7731bb14f897e450e4
diff --git a/bpf_progs/offload.c b/bpf_progs/offload.c
index 95765ff..7cec065 100644
--- a/bpf_progs/offload.c
+++ b/bpf_progs/offload.c
@@ -22,6 +22,9 @@
 
 DEFINE_BPF_MAP(tether_ingress_map, HASH, TetherIngressKey, TetherIngressValue, 64)
 
+// Tethering stats, indexed by upstream interface.
+DEFINE_BPF_MAP(tether_stats_map, HASH, uint32_t, TetherStatsValue, 16)
+
 SEC("schedcls/ingress/tether_ether")
 int sched_cls_ingress_tether_ether(struct __sk_buff* skb) {
     // TODO