sctp: use common head of addr parameter to access member in addr-unrelated code

The 'p' member of struct sctp_paramhdr is common part for
IPv4 addr parameter and IPv6 addr parameter in union sctp_addr_param.

For addr-related code, use specified addr parameter.
Otherwise, use common header to access type/length member.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>