selftests: net: udpgso_bench: Fix racing bug between the rx/tx programs

"udpgro_bench.sh" invokes udpgso_bench_rx/udpgso_bench_tx programs
subsequently and while doing so, there is a chance that the rx one is not
ready to accept socket connections. This racing bug could fail the test
with at least one of the following:

./udpgso_bench_tx: connect: Connection refused
./udpgso_bench_tx: sendmsg: Connection refused
./udpgso_bench_tx: write: Connection refused

This change addresses this by making udpgro_bench.sh wait for the rx
program to be ready before firing off the tx one - up to a 10s timeout.

Fixes: 3a687bef148d ("selftests: udp gso benchmark")
Signed-off-by: Andrei Gherzan <[e-mail]>
Cc: Paolo Abeni <[e-mail]>
Cc: Willem de Bruijn <[e-mail]>
Reviewed-by: Willem de Bruijn <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

tools/testing/selftests/net/udpgso_bench.sh

···
 readonly YELLOW='\033[0;33m'
 readonly RED='\033[0;31m'
 readonly NC='\033[0m' # No Color
+readonly TESTPORT=8000
 
 readonly KSFT_PASS=0
 readonly KSFT_FAIL=1
···
 
 run_one() {
 	local -r args=$@
+	local nr_socks=0
+	local i=0
+	local -r timeout=10
 
-	./udpgso_bench_rx &
-	./udpgso_bench_rx -t &
+	./udpgso_bench_rx -p "$TESTPORT" &
+	./udpgso_bench_rx -p "$TESTPORT" -t &
 
-	./udpgso_bench_tx ${args}
+	# Wait for the above test program to get ready to receive connections.
+	while [ "$i" -lt "$timeout" ]; do
+		nr_socks="$(ss -lnHi | grep -c "\*:${TESTPORT}")"
+		[ "$nr_socks" -eq 2 ] && break
+		i=$((i + 1))
+		sleep 1
+	done
+	if [ "$nr_socks" -ne 2 ]; then
+		echo "timed out while waiting for udpgso_bench_rx"
+		exit 1
+	fi
+
+	./udpgso_bench_tx -p "$TESTPORT" ${args}
 }
 
 run_in_netns() {