MFC r357233:

epair: Do not abuse params to register the second interface

if_epair used the 'params' argument to pass a pointer to the b interface
through if_clone_create().
This pointer can be controlled by userspace, which means it could be abused to
trigger a panic. While this requires PRIV_NET_IFCREATE
privileges those are assigned to vnet jails, which means that vnet jails
could panic the system.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>