PM / Freezer: Remove the "userspace only" constraint from freezer[_do_not]_count()

At present, the functions freezer_count() and freezer_do_not_count()
impose the restriction that they are effective only for userspace processes.
However, now, these functions have found more utility than originally
intended by the commit which introduced it: ba96a0c8 (freezer:
fix vfork problem). And moreover, even the vfork issue actually does not
need the above restriction in these functions.

So, modify these functions to make them work even for kernel threads, so
that they can be used at other places in the kernel, where the userspace
restriction doesn't apply.
Suggested-by: NOleg Nesterov <oleg@redhat.com>
Suggested-by: NTejun Heo <tj@kernel.org>
Acked-by: NTejun Heo <tj@kernel.org>
Reviewed-by: NOleg Nesterov <oleg@redhat.com>
Signed-off-by: NSrivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>

include/linux/freezer.h

@@ -67,33 +67,27 @@ static inline bool cgroup_freezing(struct task_struct *task)
  * appropriately in case the child has exited before the freezing of tasks is
  * complete.  However, we don't want kernel threads to be frozen in unexpected
  * places, so we allow them to block freeze_processes() instead or to set
- * PF_NOFREEZE if needed and PF_FREEZER_SKIP is only set for userland vfork
- * parents.  Fortunately, in the ____call_usermodehelper() case the parent won't
- * really block freeze_processes(), since ____call_usermodehelper() (the child)
- * does a little before exec/exit and it can't be frozen before waking up the
- * parent.
+ * PF_NOFREEZE if needed. Fortunately, in the ____call_usermodehelper() case the
+ * parent won't really block freeze_processes(), since ____call_usermodehelper()
+ * (the child) does a little before exec/exit and it can't be frozen before
+ * waking up the parent.
  */
 
-/*
- * If the current task is a user space one, tell the freezer not to count it as
- * freezable.
- */
+
+/* Tell the freezer not to count the current task as freezable. */
 static inline void freezer_do_not_count(void)
 {
-	if (current->mm)
 	current->flags |= PF_FREEZER_SKIP;
 }
 
 /*
- * If the current task is a user space one, tell the freezer to count it as
- * freezable again and try to freeze it.
+ * Tell the freezer to count the current task as freezable again and try to
+ * freeze it.
  */
 static inline void freezer_count(void)
 {
-	if (current->mm) {
 	current->flags &= ~PF_FREEZER_SKIP;
 	try_to_freeze();
-	}
 }
 
 /*