selftests/bpf: Add new bpf_iter context structs to fix build on old kernels

Add bpf_iter__bpf_map_elem and bpf_iter__bpf_sk_storage_map to bpf_iter.h.

Fixes: 3b1c420b ("selftests/bpf: Add a test for bpf sk_storage_map iterator")
Fixes: 2a7c2fff ("selftests/bpf: Add test for bpf hash map iterators")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200727233345.1686358-1-andriin@fb.com