pad: Reset not-linked last flow return on reconfigure events

pad: Reset not-linked last flow return on reconfigure events

The pad might actually be linked now and we'd only find out by actually pushing
a buffer downstream again.

The last flow return is used by GstFlowCombiner to detect if all pads are not
linked, and not resetting this when re-linking creates a race condition when
switching between pads where all pads are temporarily considered not linked.

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/3836

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8224>