udplite: make socketoption available on IPv6 sockets

This patch allows the IPPROTO_UDPLITE-level socket options
UDPLITE_SEND_CSCOV and UDPLITE_RECV_CSCOV to be used on
AF_INET6 sockets in addition to AF_INET sockets.

Reviewed by: ae, rscheff
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42430

(cherry picked from commit 03c3a70abe5e9fa259b954de78ae69229fa9c99f)