kselftests: cgroup: Avoid the reuse of fd after it is deallocated

It is necessary to set fd to -1 when inotify_add_watch() fails in
cg_prepare_for_wait. Otherwise the fd which has been closed in
cg_prepare_for_wait may be misused in other functions such as
cg_enter_and_wait_for_frozen and cg_freeze_wait.

Fixes: 5313bfe425c8 ("selftests: cgroup: add freezer controller self-tests")
Signed-off-by: Hewenliang <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

tools/testing/selftests/cgroup/test_freezer.c

···
 	if (ret == -1) {
 		debug("Error: inotify_add_watch() failed\n");
 		close(fd);
+		fd = -1;
 	}
 
 	return fd;