IPv4 multicast: fix netstat -g

The vif structure includes fields at the end which are #ifdef KERNEL,
causing a mismatch between the structure sizes between kernel and
user level. netstat -g failed with an ENOMEM on the sysctl to fetch
the vif table. Change the vif sysctl code in ip_mroute to copy out
only the user-level-visible portion of each table entry.

Reviewed by: bz, wma
Differential Revision: https://reviews.freebsd.org/D34627