Revert "Revert "sctp: Fix bundling of SHUTDOWN with COOKIE-ACK""

This reverts commit 7e9269a5.

As Jere notice, commit 35b4f244

 ("sctp: do asoc update earlier
in sctp_sf_do_dupcook_a") only keeps the SHUTDOWN and COOKIE-ACK
with the same asoc, not transport. So we have to bring this patch
back.

Reported-by: Jere Leppänen <jere.leppanen@nokia.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>