cred: use correct cred accessor with regards to rcu read lock

Commit "userns: Convert setting and getting uid and gid system calls to use
kuid and kgid has modified the accessors in wait_task_continued() and
wait_task_stopped() to use __task_cred() instead of task_uid().

__task_cred() assumes that we're inside a rcu read lock, which is untrue
for these two functions.

Modify it to use task_uid() instead.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>