sched: fix nr_uninterruptible accounting of frozen tasks really

commit e3c8ca83 (sched: do not count frozen tasks toward load) broke
the nr_uninterruptible accounting on freeze/thaw. On freeze the task
is excluded from accounting with a check for (task->flags &
PF_FROZEN), but that flag is cleared before the task is thawed. So
while we prevent that the task with state TASK_UNINTERRUPTIBLE
is accounted to nr_uninterruptible on freeze we decrement
nr_uninterruptible on thaw.

Use a separate flag which is handled by the freezing task itself. Set
it before calling the scheduler with TASK_UNINTERRUPTIBLE state and
clear it after we return from frozen state.

Cc: <stable@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>