selftests/net: Don't forget to close nsfd after switch_save_ns()

The switch_save_ns() helper suppose to help switching to another
namespace for some action and to return back to original namespace.

The fd should be closed.
Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20240823-tcp-ao-selftests-upd-6-12-v4-5-05623636fe8c@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/tcp_ao/lib/setup.c

@@ -142,6 +142,13 @@ int switch_save_ns(int new_ns)
 	return ret;
 }
 
+void switch_close_ns(int fd)
+{
+	if (setns(fd, CLONE_NEWNET))
+		test_error("setns()");
+	close(fd);
+}
+
 static int nsfd_outside	= -1;
 static int nsfd_parent	= -1;
 static int nsfd_child	= -1;
@@ -296,7 +303,7 @@ static bool is_optmem_namespaced(void)
 		int old_ns = switch_save_ns(nsfd_child);
 
 		optmem_ns = !access(optmem_file, F_OK);
-		switch_ns(old_ns);
+		switch_close_ns(old_ns);
 	}
 	return !!optmem_ns;
 }
@@ -317,7 +324,7 @@ size_t test_get_optmem(void)
 		test_error("can't read from %s", optmem_file);
 	fclose(foptmem);
 	if (!is_optmem_namespaced())
-		switch_ns(old_ns);
+		switch_close_ns(old_ns);
 	return ret;
 }
 
@@ -339,7 +346,7 @@ static void __test_set_optmem(size_t new, size_t *old)
 		test_error("can't write %zu to %s", new, optmem_file);
 	fclose(foptmem);
 	if (!is_optmem_namespaced())
-		switch_ns(old_ns);
+		switch_close_ns(old_ns);
 }
 
 static void test_revert_optmem(void)