net: bpf: Implement bpf iterator for udp

The bpf iterator for udp is implemented. Both udp4 and udp6
sockets will be traversed. It is up to bpf program to
filter for udp4 or udp6 only, or both families of sockets.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200623230813.3988404-1-yhs@fb.com