sctp: factor out sctp_connect_add_peer

In this function factored out from sctp_sendmsg_new_asoc() and
__sctp_connect(), it adds a peer with the other addr into the
asoc after this asoc is created with the 1st addr.

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