selftests: net: explicitly wait for listener ready

The UDP GRO forwarding test still hard-code an arbitrary pause
to wait for the UDP listener becoming ready in background.

That causes sporadic failures depending on the host load.

Replace the sleep with the existing helper waiting for the desired
port being exposed.

Fixes: a062260a ("selftests: net: add UDP GRO forwarding self-tests")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/4d58900fb09cef42749cfcf2ad7f4b91a97d225c.1706131762.git.pabeni@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/udpgro_fwd.sh

 #!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
+source net_helper.sh
+
 BPF_FILE="xdp_dummy.o"
 readonly BASE="ns-$(mktemp -u XXXXXX)"
 readonly SRC=2
@@ -119,7 +121,7 @@ run_test() {
 	ip netns exec $NS_DST $ipt -A INPUT -p udp --dport 8000
 	ip netns exec $NS_DST ./udpgso_bench_rx -C 1000 -R 10 -n 10 -l 1300 $rx_args &
 	local spid=$!
-	sleep 0.1
+	wait_local_port_listen "$NS_DST" 8000 udp
 	ip netns exec $NS_SRC ./udpgso_bench_tx $family -M 1 -s 13000 -S 1300 -D $dst
 	local retc=$?
 	wait $spid
@@ -168,7 +170,7 @@ run_bench() {
 	ip netns exec $NS_DST bash -c "echo 2 > /sys/class/net/veth$DST/queues/rx-0/rps_cpus"
 	ip netns exec $NS_DST taskset 0x2 ./udpgso_bench_rx -C 1000 -R 10  &
 	local spid=$!
-	sleep 0.1
+	wait_local_port_listen "$NS_DST" 8000 udp
 	ip netns exec $NS_SRC taskset 0x1 ./udpgso_bench_tx $family -l 3 -S 1300 -D $dst
 	local retc=$?
 	wait $spid