fork: call complete_vfork_done() after clearing child_tid and flushing rss-counters

Child should wake up the parent from vfork() only after finishing all
operations with shared mm. There is no sense in using
CLONE_CHILD_CLEARTID together with CLONE_VFORK, but it looks more accurate
now.

Signed-off-by: Konstantin Khlebnikov <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: KAMEZAWA Hiroyuki <[e-mail]>
Cc: Konstantin Khlebnikov <[e-mail]>
Cc: Markus Trippelsdorf <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/fork.c

···
 	/* Get rid of any cached register state */
 	deactivate_mm(tsk, mm);
 
-	if (tsk->vfork_done)
-		complete_vfork_done(tsk);
-
 	/*
 	 * If we're exiting normally, clear a user-space tid field if
 	 * requested.  We leave this alone when dying by signal, to leave
···
 		}
 		tsk->clear_child_tid = NULL;
 	}
+
+	/*
+	 * All done, finally we can wake up parent and return this mm to him.
+	 * Also kthread_stop() uses this completion for synchronization.
+	 */
+	if (tsk->vfork_done)
+		complete_vfork_done(tsk);
 }
 
 /*