selftests: net: switch to socat in the GSO GRE test

Commit a985442fdecb ("selftests: net: properly support IPv6 in GSO GRE test")
is not compatible with:

Ncat: Version 7.80 ( https://nmap.org/ncat )

(which is distributed with Fedora/Red Hat), tests fail with:

nc: invalid option -- 'N'

Let's switch to socat which is far more dependable.

Fixes: 025efa0a82df ("selftests: add simple GSO GRE test")
Fixes: a985442fdecb ("selftests: net: properly support IPv6 in GSO GRE test")
Tested-by: Andrea Righi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/gre_gso.sh

···
 	local addr=$2
 	local proto=$3
 
-	$NS_EXEC nc $proto -kl $port >/dev/null &
+	[ "$proto" == 6 ] && addr="[$addr]"
+
+	$NS_EXEC socat - tcp${proto}-listen:$port,reuseaddr,fork >/dev/null &
 	PID=$!
 	while ! $NS_EXEC ss -ltn | grep -q $port; do ((i++)); sleep 0.01; done
 
-	cat $TMPFILE | timeout 1 nc $proto -N $addr $port
+	cat $TMPFILE | timeout 1 socat -u STDIN TCP:$addr:$port
 	log_test $? 0 "$name - copy file w/ TSO"
 
 	ethtool -K veth0 tso off
 
-	cat $TMPFILE | timeout 1 nc $proto -N $addr $port
+	cat $TMPFILE | timeout 1 socat -u STDIN TCP:$addr:$port
 	log_test $? 0 "$name - copy file w/ GSO"
 
 	ethtool -K veth0 tso on
···
 
 	sleep 2
 
-	gre_gst_test_checks GREv6/v4 172.16.2.2
-	gre_gst_test_checks GREv6/v6 2001:db8:1::2 -6
+	gre_gst_test_checks GREv6/v4 172.16.2.2 4
+	gre_gst_test_checks GREv6/v6 2001:db8:1::2 6
 
 	cleanup
 }
···
 	exit $ksft_skip
 fi
 
-if [ ! -x "$(command -v nc)" ]; then
-	echo "SKIP: Could not run test without nc tool"
+if [ ! -x "$(command -v socat)" ]; then
+	echo "SKIP: Could not run test without socat tool"
 	exit $ksft_skip
 fi