selftests/bpf: Add write tests for sk local storage map iterator

Add test to validate the overwrite of sock local storage map value in
map iterator and another one to ensure out-of-bound value writing is
rejected.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/r/20220810080538.1845898-9-houtao@huaweicloud.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>