selftests/bpf: add sockopt test that exercises sk helpers

socktop test that introduces new SOL_CUSTOM sockopt level and
stores whatever users sets in sk storage. Whenever getsockopt
is called, the original value is retrieved.

v9:
* SO_SNDBUF example to override user-supplied buffer

v7:
* use retval=0 and optlen-1

v6:
* test 'ret=1' use-case as well (Alexei Starovoitov)

v4:
* don't call bpf_sk_fullsock helper

v3:
* drop (__u8 *)(long) casts for optval{,_end}

v2:
* new test

Cc: Andrii Nakryiko <andriin@fb.com>
Cc: Martin Lau <kafai@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>