freezer: exec should clear PF_NOFREEZE along with PF_KTHREAD

commit b40a79591ca918e7b91b0d9b6abd5d00f2e88c19 upstream.

flush_old_exec() clears PF_KTHREAD but forgets about PF_NOFREEZE.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
[bwh: Backported to 3.2: PF_FORKNOEXEC is cleared elsewhere]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>