Freezer: Fix a race during freezing of TASK_STOPPED tasks

After calling freeze_task(), try_to_freeze_tasks() see whether the
task is stopped or traced and if so, considers it to be frozen;
however, nothing guarantees that either the task being frozen sees
TIF_FREEZE or the freezer sees TASK_STOPPED -> TASK_RUNNING
transition.  The task being frozen may wake up and not see TIF_FREEZE
while the freezer fails to notice the transition and believes the task
is still stopped.

This patch fixes the race by making freeze_task() always go through
fake_signal_wake_up() for applicable tasks.  The function goes through
the target task's scheduler lock and thus guarantees that either the
target sees TIF_FREEZE or try_to_freeze_task() sees TASK_RUNNING.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
cgroup: speed up access to cgroupfs mounted entries in /proc/mounts