[PATCH bpf-next] selftests/bpf: Fix strncpy() fortify warning

[Rong Tao <rtoax@xxxxxxxxxxx>]

From: Rong Tao <rongtao@xxxxxxxx>

Replace strncpy() with strncat(), strncat() leaves the dst string zero
terminated. Compile samples/bpf warning:

$ cd samples/bpf
$ make
...
In function ‘__enable_controllers’:
samples/bpf/../../tools/testing/selftests/bpf/cgroup_helpers.c:80:17: warning: ‘strncpy’ specified bound 4097 equals destination size [-Wstringop-truncation]
   80 |                 strncpy(enable, controllers, sizeof(enable));
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Rong Tao <rongtao@xxxxxxxx>
---
 tools/testing/selftests/bpf/cgroup_helpers.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/cgroup_helpers.c b/tools/testing/selftests/bpf/cgroup_helpers.c
index a70e873b267e..912e6522c7c5 100644
--- a/tools/testing/selftests/bpf/cgroup_helpers.c
+++ b/tools/testing/selftests/bpf/cgroup_helpers.c
@@ -77,7 +77,8 @@ static int __enable_controllers(const char *cgroup_path, const char *controllers
 		enable[len] = 0;
 		close(fd);
 	} else {
-		strncpy(enable, controllers, sizeof(enable) - 1);
+		enable[0] = '\0';
+		strncat(enable, controllers, sizeof(enable) - 1);
 	}
 
 	snprintf(path, sizeof(path), "%s/cgroup.subtree_control", cgroup_path);
-- 
2.31.1