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

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

[ Upstream commit d671fa6393d6788fc65555d4643b71cb3a361f36 ]

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 <hewenliang4@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>