sctp: add SCTP_CURRENT_ASSOC for SCTP_STREAM_SCHEDULER_VALUE sockopt

SCTP_STREAM_SCHEDULER_VALUE is a special one, as its value is not
save in sctp_sock, but only in asoc. So only SCTP_CURRENT_ASSOC
reserved assoc_id can be used in sctp_setsockopt_scheduler_value.

This patch adds SCTP_CURRENT_ASOC support for
SCTP_STREAM_SCHEDULER_VALUE.

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