selftests: bpf: Add selftest for __sk_buff tstamp

Make sure BPF_PROG_TEST_RUN accepts tstamp and exports any
modifications that BPF program does.
Signed-off-by: NStanislav Fomichev <sdf@google.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Acked-by: NMartin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20191015183125.124413-2-sdf@google.com

tools/testing/selftests/bpf/prog_tests/skb_ctx.c

@@ -10,6 +10,7 @@ void test_skb_ctx(void)
 		.cb[3] = 4,
 		.cb[4] = 5,
 		.priority = 6,
+		.tstamp = 7,
 	};
 	struct bpf_prog_test_run_attr tattr = {
 		.data_in = &pkt_v4,
@@ -86,4 +87,8 @@ void test_skb_ctx(void)
 		   "ctx_out_priority",
 		   "skb->priority == %d, expected %d\n",
 		   skb.priority, 7);
+	CHECK_ATTR(skb.tstamp != 8,
+		   "ctx_out_tstamp",
+		   "skb->tstamp == %lld, expected %d\n",
+		   skb.tstamp, 8);
 }

tools/testing/selftests/bpf/progs/test_skb_ctx.c

@@ -16,6 +16,7 @@ int process(struct __sk_buff *skb)
 		skb->cb[i]++;
 	}
 	skb->priority++;
+	skb->tstamp++;
 
 	return 0;
 }