sctp: Add Auto-ASCONF support (core).

SCTP reconfigure the IP addresses in the association by using
ASCONF chunks as mentioned in RFC5061.  For example, we can
start to use the newly configured IP address in the existing
association.  This patch implements automatic ASCONF operation
in the SCTP stack with address events in the host computer,
which is called auto_asconf.

Signed-off-by: Michio Honda <micchie@sfc.wide.ad.jp>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Acked-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>