ng_iface(4): Set the current VNET before calling netisr_dispatch().

This is normally handled by a netgraph thread, but netgraph messages may
be dispatched directly to a node, in which case no VNET is set before
ng_iface calls into the network stack. Netgraph could probably handle
this more generally, but for now just be sure to set the current VNET in
ng_iface.

PR: 242406
Tested by: Michael Muenz <m.muenz@gmail.com>
Reviewed by: Lutz Donnerhacke
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25788