af_unix: Implement ->read_sock() for sockmap

Implement ->read_sock() for AF_UNIX datagram socket, it is
pretty much similar to udp_read_sock().

Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210704190252.11866-4-xiyou.wangcong@gmail.com