Document/enforce a bug in udp_dump_one.

udp_dump_one passes arguments to __udp[46]_lib_lookup in the
wrong order. Thus, to find a UDP socket from an inet_diag_req_v2,
userspace must swap src and dst addresses and ports.

Upstream has stated that this cannot be fixed any more for
backwards compatibility reasons.

Test: test passes on android-4.9
Change-Id: I98dfaa34608bb8291a21be4da03e1ad6b4039899