sctp: flush out queue once assoc state falls into SHUTDOWN_PENDING

This patch is to flush out queue when assoc state falls into
SHUTDOWN_PENDING if there are still chunks in it, so that the
data can be sent out as soon as possible before sending SHUTDOWN
chunk.

When sctp supports MSG_MORE flag in next patch, this improvement
can also solve the problem that the chunks with MSG_MORE flag
may be stuck in queue when closing an assoc.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>