bpf: Support bpf_(get|set)sockopt() in bpf unix iter.

This patch makes bpf_(get|set)sockopt() available when iterating AF_UNIX
sockets.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Link: https://lore.kernel.org/r/20220113002849.4384-4-kuniyu@amazon.co.jp


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