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>

kernel/exit.c

@@ -1427,7 +1427,7 @@ static int wait_task_stopped(struct wait_opts *wo,
 	if (!unlikely(wo->wo_flags & WNOWAIT))
 		*p_code = 0;
 
-	uid = from_kuid_munged(current_user_ns(), __task_cred(p)->uid);
+	uid = from_kuid_munged(current_user_ns(), task_uid(p));
 unlock_sig:
 	spin_unlock_irq(&p->sighand->siglock);
 	if (!exit_code)
@@ -1500,7 +1500,7 @@ static int wait_task_continued(struct wait_opts *wo, struct task_struct *p)
 	}
 	if (!unlikely(wo->wo_flags & WNOWAIT))
 		p->signal->flags &= ~SIGNAL_STOP_CONTINUED;
-	uid = from_kuid_munged(current_user_ns(), __task_cred(p)->uid);
+	uid = from_kuid_munged(current_user_ns(), task_uid(p));
 	spin_unlock_irq(&p->sighand->siglock);
 
 	pid = task_pid_vnr(p);