ipf_frname_vfy() bounds checks fr->fr_names indicies and proceeds to
copy the interface name using FR_NAME(). The ipfilter kernel module
receives the correct inputs from the ipfilter userland tools. However
a rogue actor may call the ipfilter ioctl passing out of band data.
We need to protect ourselves from this.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
MFC after: 1 week