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.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>