sctp: define SCTP_SS_DEFAULT for Stream schedulers

According to rfc8260#section-4.3.2, SCTP_SS_DEFAULT is required to
defined as SCTP_SS_FCFS or SCTP_SS_RR.

SCTP_SS_FCFS is used for SCTP_SS_DEFAULT's value in this patch.

Fixes: 5bbbbe32

 ("sctp: introduce stream scheduler foundations")
Reported-by: Jianwen Ji <jiji@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>