ip_mroute: fix the viftable export sysctl

It seems that in r354857 I got more than one thing wrong.
Convert the SYSCTL_OPAQUE to a SYSCTL_PROC to properly export the these
days allocated and not longer static per-vnet viftable array.
This fixes a problem with netstat -g which would show bogus information
for the IPv4 Virtual Interface Table.

PR: 246626
Reported by: Ozkan KIRIK (ozkan.kirik gmail.com)
MFC after: 3 days