ipfilter: Add ipf_frname_vfy()

[cy]

Summary

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 <[e-mail]>
MFC after: 1 week

[cy]

This is no longer needed as the validation is now to be performed in frrequest().