sctp: Fix SHUTDOWN CTSN Ack in the peer restart case

When starting shutdown in sctp_sf_do_dupcook_a(), get the value for
SHUTDOWN Cumulative TSN Ack from the new association, which is
reconstructed from the cookie, instead of the old association, which
the peer doesn't have anymore.

Otherwise the SHUTDOWN is either ignored or replied to with an ABORT
by the peer because CTSN Ack doesn't match the peer's Initial TSN.

Fixes: bdf6fa52

 ("sctp: handle association restarts when the socket is closed.")
Signed-off-by: Jere Leppänen <jere.leppanen@nokia.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>