linux: add sysctl to pass untranslated interface names

Reviewed by: kib
Approved by: re(gjb)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D33792

(cherry picked from commit 1f70a85b4cbc3ad19cec4a390e8754e54815be85)
(cherry picked from commit c7655e1f3671a9ce7d963cb577b4548173469053)