linux(4): Consolidate a FreeBSD interface names translation code

We have some amount of interface names translation functions which are
differs by bugs implementation. Consolidates it in a one place.

Fixup loopback interface names translation and use ifnet methods and
accessors, where possible.

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D38714
MFC after: 3 days
X-MFC with: 32fdc75fe7