sctp: factor out sctp_sendmsg_get_daddr from sctp_sendmsg

This patch is to move the codes for trying to get daddr from
msg->msg_name into sctp_sendmsg_get_daddr.

Note that after adding 'daddr', 'to' and 'msg_name' can be
deleted.

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