selftest/bpf: Test batching and bpf_(get|set)sockopt in bpf unix iter.

This patch adds a test for the batching and bpf_(get|set)sockopt in bpf
unix iter.

It does the following.

  1. Creates an abstract UNIX domain socket
  2. Call bpf_setsockopt()
  3. Call bpf_getsockopt() and save the value
  4. Call setsockopt()
  5. Call getsockopt() and save the value
  6. Compare the saved values

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Link: https://lore.kernel.org/r/20220113002849.4384-5-kuniyu@amazon.co.jp


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