MFC r357234, r357365, r357375:

tests: Test for an epair panic

if_epair abused the ifr_data field to insert its second interface in
IFC_IFLIST. If userspace provides a value for ifr_data it would get
dereferenced by the kernel leading to a panic.

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