collectpads: Use private pad list in set_flushing_unlocked

collectpads: Use private pad list in set_flushing_unlocked

pads->data is the public list. It is dynamically rebuilt at each call to
check_collected, in check_pads to be specific. When you add a pad and
collectpads have been started, it is not added to the public list.

Thus there exists a possible race where :

1) You would add a pad to collectpads while running.
2) You set collectpads to flushing before check_collected has been called again
-> the pad is not set to flushing
3) the pad starts pushing data as downstream might not be prepared, in the case
of adder it then returns FLOW_FLUSHING.
4) elements like demuxers, when they get a FLOW_FLUSHING, stop their tasks,
never to be seen again.

https://bugzilla.gnome.org/show_bug.cgi?id=708636