selftests/bpf: Fix flaky selftest lwt_redirect/lwt_reroute

Recently, when running './test_progs -j', I occasionally hit the
following errors:

  test_lwt_redirect:PASS:pthread_create 0 nsec
  test_lwt_redirect_run:FAIL:netns_create unexpected error: 256 (errno 0)
  #142/2   lwt_redirect/lwt_redirect_normal_nomac:FAIL
  #142     lwt_redirect:FAIL
  test_lwt_reroute:PASS:pthread_create 0 nsec
  test_lwt_reroute_run:FAIL:netns_create unexpected error: 256 (errno 0)
  test_lwt_reroute:PASS:pthread_join 0 nsec
  #143/2   lwt_reroute/lwt_reroute_qdisc_dropped:FAIL
  #143     lwt_reroute:FAIL

The netns_create() definition looks like below:

  #define NETNS "ns_lwt"
  static inline int netns_create(void)
  {
        return system("ip netns add " NETNS);
  }

One possibility is that both lwt_redirect and lwt_reroute create
netns with the same name "ns_lwt" which may cause conflict. I tried
the following example:
  $ sudo ip netns add abc
  $ echo $?
  0
  $ sudo ip netns add abc
  Cannot create namespace file "/var/run/netns/abc": File exists
  $ echo $?
  1
  $

The return code for above netns_create() is 256. The internet search
suggests that the return value for 'ip netns add ns_lwt' is 1, which
matches the above 'sudo ip netns add abc' example.

This patch tried to use different netns names for two tests to avoid
'ip netns add <name>' failure.

I ran './test_progs -j' 10 times and all succeeded with
lwt_redirect/lwt_reroute tests.
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20240205052914.1742687-1-yonghong.song@linux.dev

tools/testing/selftests/bpf/prog_tests/lwt_helpers.h

@@ -27,8 +27,6 @@
 			}                                                     \
 	})
 
-#define NETNS "ns_lwt"
-
 static inline int netns_create(void)
 {
 	return system("ip netns add " NETNS);

tools/testing/selftests/bpf/prog_tests/lwt_redirect.c

@@ -54,6 +54,7 @@
 #include <stdbool.h>
 #include <stdlib.h>
 
+#define NETNS "ns_lwt_redirect"
 #include "lwt_helpers.h"
 #include "test_progs.h"
 #include "network_helpers.h"

tools/testing/selftests/bpf/prog_tests/lwt_reroute.c

@@ -48,6 +48,7 @@
  *  For case 2, force UDP packets to overflow fq limit. As long as kernel
  *  is not crashed, it is considered successful.
  */
+#define NETNS "ns_lwt_reroute"
 #include "lwt_helpers.h"
 #include "network_helpers.h"
 #include <linux/net_tstamp.h>