freezer: rename thaw_process() to __thaw_task() and simplify the implementation

thaw_process() now has only internal users - system and cgroup
freezers.  Remove the unnecessary return value, rename, unexport and
collapse __thaw_process() into it.  This will help further updates to
the freezer code.

-v3: oom_kill grew a use of thaw_process() while this patch was
     pending.  Convert it to use __thaw_task() for now.  In the longer
     term, this should be handled by allowing tasks to die if killed
     even if it's frozen.

-v2: minor style update as suggested by Matt.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Paul Menage <menage@google.com>
Cc: Matt Helsley <matthltc@us.ibm.com>

kernel/power/process.c

@@ -186,7 +186,7 @@ static void thaw_tasks(bool nosig_only)
 		if (cgroup_freezing_or_frozen(p))
 			continue;
 
-		thaw_process(p);
+		__thaw_task(p);
 	} while_each_thread(g, p);
 	read_unlock(&tasklist_lock);
 }