sctp: change sk state to CLOSED instead of CLOSING in sctp_sock_migrate

Commit d46e416c ("sctp: sctp should change socket state when
shutdown is received") may set sk_state CLOSING in sctp_sock_migrate,
but inet_accept doesn't allow the sk_state other than ESTABLISHED/
CLOSED for sctp. So we will change sk_state to CLOSED, instead of
CLOSING, as actually sk is closed already there.

Fixes: d46e416c

 ("sctp: sctp should change socket state when shutdown is received")
Reported-by: Ye Xiaolong <xiaolong.ye@intel.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>