selftests/bpf/test_tcp_check_syncookie: use temp netns for testing

Use temp netns instead of hard code name for testing in case the
netns already exists.
Signed-off-by: NHangbin Liu <liuhangbin@gmail.com>
Acked-by: NLorenz Bauer <lmb@cloudflare.com>
Link: https://lore.kernel.org/r/20220125081717.1260849-6-liuhangbin@gmail.comSigned-off-by: NAlexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/test_tcp_check_syncookie.sh

@@ -4,6 +4,7 @@
 # Copyright (c) 2019 Cloudflare
 
 set -eu
+readonly NS1="ns1-$(mktemp -u XXXXXX)"
 
 wait_for_ip()
 {
@@ -28,12 +29,12 @@ get_prog_id()
 
 ns1_exec()
 {
-	ip netns exec ns1 "$@"
+	ip netns exec ${NS1} "$@"
 }
 
 setup()
 {
-	ip netns add ns1
+	ip netns add ${NS1}
 	ns1_exec ip link set lo up
 
 	ns1_exec sysctl -w net.ipv4.tcp_syncookies=2