sctpenc: Fix potential shutdown deadlock

sctpenc: Fix potential shutdown deadlock

When transitioning from state PAUSED to READY, the sctpenc element
could previously be stuck in an endless loop trying to resend data
in case the underlying sctp stream was in the process of
resetting. usrsctp_sendv() would repeatedly return EAGAIN with the
result that 0 bytes were sent and then sctpenc would retry forever.

To bring sctpenc out of the resend loop we just need to inform the
sink pad that it is flushing, which is already done for the associated
data queue, but we also need to set the bools associated with the
sinkpads that are used as the loop criterion.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4636>