udp_stream: collect TX stats, too
udp_stream: collect TX stats, too

On UDP flows the xmitted tput can be greater than the received
one, so collect stats on TX side, too.

Signed-off-by: Paolo Abeni <6e4ffc2e3438feb7acb34f6b67e6ae17e700099c@redhat.com>

jsitnicki/rushit,jsitnicki/rushit