samples/bpf: unit test for BPF_MAP_TYPE_PERCPU_HASH

A sanity test for BPF_MAP_TYPE_PERCPU_HASH.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>