sctp: factor out sctp_sendmsg_update_sinfo from sctp_sendmsg

This patch is to move the codes for trying to get sinfo from
asoc into sctp_sendmsg_update_sinfo.

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