net: bpf: Add bpf_seq_afinfo in udp_iter_state

Similar to tcp_iter_state, a new field bpf_seq_afinfo is
added to udp_iter_state to provide bpf udp iterator
afinfo.

This does not change /proc/net/{udp, udp6} behavior. But
it enables bpf iterator to avoid get afinfo from PDE_DATA
and iterate through all udp and udp6 sockets in one pass.

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/20200623230812.3988347-1-yhs@fb.com