bpf: Add hashtab support for bpf_for_each_map_elem() helper

This patch added support for hashmap, percpu hashmap,
lru hashmap and percpu lru hashmap.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210226204927.3885020-1-yhs@fb.com