samples/bpf: unit test for BPF_MAP_TYPE_PERCPU_ARRAY

A sanity test for BPF_MAP_TYPE_PERCPU_ARRAY

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>