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: 5313bfe4

 ("selftests: cgroup: add freezer controller self-tests")
Signed-off-by: Hewenliang <hewenliang4@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>