selftests/bpf: test map percpu stats

Add a new map test, map_percpu_stats.c, which is checking the correctness of
map's percpu elements counters.  For supported maps the test upserts a number
of elements, checks the correctness of the counters, then deletes all the
elements and checks again that the counters sum drops down to zero.

The following map types are tested:

    * BPF_MAP_TYPE_HASH, BPF_F_NO_PREALLOC
    * BPF_MAP_TYPE_PERCPU_HASH, BPF_F_NO_PREALLOC
    * BPF_MAP_TYPE_HASH,
    * BPF_MAP_TYPE_PERCPU_HASH,
    * BPF_MAP_TYPE_LRU_HASH
    * BPF_MAP_TYPE_LRU_PERCPU_HASH
    * BPF_MAP_TYPE_LRU_HASH, BPF_F_NO_COMMON_LRU
    * BPF_MAP_TYPE_LRU_PERCPU_HASH, BPF_F_NO_COMMON_LRU
    * BPF_MAP_TYPE_HASH_OF_MAPS
Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Acked-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20230706133932.45883-6-aspsk@isovalent.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Isovalent */
+
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+__u32 target_id;
+
+__s64 bpf_map_sum_elem_count(struct bpf_map *map) __ksym;
+
+SEC("iter/bpf_map")
+int dump_bpf_map(struct bpf_iter__bpf_map *ctx)
+{
+	struct seq_file *seq = ctx->meta->seq;
+	struct bpf_map *map = ctx->map;
+
+	if (map && map->id == target_id)
+		BPF_SEQ_PRINTF(seq, "%lld", bpf_map_sum_elem_count(map));
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";