The bpf iterators for array and percpu array
are implemented. Similar to hash maps, for percpu
array map, bpf program will receive values
from all cpus.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200723184115.590532-1-yhs@fb.com