selftests/bpf: Add a test for bpf sk_storage_map iterator

Added one test for bpf sk_storage_map_iterator.
  $ ./test_progs -n 4
  ...
  #4/22 bpf_sk_storage_map:OK
  ...

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200723184122.591591-1-yhs@fb.com

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

@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include "bpf_iter.h"
+#include "bpf_tracing_net.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+struct {
+	__uint(type, BPF_MAP_TYPE_SK_STORAGE);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
+	__type(key, int);
+	__type(value, int);
+} sk_stg_map SEC(".maps");
+
+__u32 val_sum = 0;
+__u32 ipv6_sk_count = 0;
+
+SEC("iter/bpf_sk_storage_map")
+int dump_bpf_sk_storage_map(struct bpf_iter__bpf_sk_storage_map *ctx)
+{
+	struct sock *sk = ctx->sk;
+	__u32 *val = ctx->value;
+
+	if (sk == (void *)0 || val == (void *)0)
+		return 0;
+
+	if (sk->sk_family == AF_INET6)
+		ipv6_sk_count++;
+
+	val_sum += *val;
+	return 0;
+}