sctp: out_qlen should be updated when pruning unsent queue

This patch is to fix the issue that sctp_prsctp_prune_sent forgot
to update q->out_qlen when removing a chunk from unsent queue.

Fixes: 8dbdf1f5 ("sctp: implement prsctp PRIO policy")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>