sctp: add a function to verify the sctp reconf chunk

This patch is to add a function sctp_verify_reconf to do some length
check and multi-params check for sctp stream reconf according to rfc6525
section 3.1.

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